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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Sep. 30, 2017	Dec. 06, 2017	Mar. 31, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Trading Symbol	ARWR
Entity Registrant Name	ARROWHEAD PHARMACEUTICALS, INC.
Entity Central Index Key	0000879407
Current Fiscal Year End Date	--09-30
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		74,828,280
Entity Public Float			$ 137

Consolidated Balance Sheets - USD ($)	Sep. 30, 2017	Sep. 30, 2016
CURRENT ASSETS
Cash and cash equivalents	$ 24,838,567	$ 85,366,448
Accounts receivable	67,797	75,000
Prepaid expenses	867,363	1,289,923
Other current assets	1,359,638	3,771,172
Short term investments	40,769,539
TOTAL CURRENT ASSETS	67,902,904	90,502,543
Property and equipment, net	15,513,019	15,386,761
Intangible assets, net	20,464,439	22,164,868
Other assets	141,918	122,333
TOTAL ASSETS	104,022,280	128,176,505
CURRENT LIABILITIES
Accounts payable	4,076,514	12,232,906
Accrued expenses	4,564,507	4,587,467
Accrued payroll and benefits	3,399,679	3,969,706
Deferred rent	440,580	440,580
Deferred revenue	5,269,741	2,569,792
Derivative liabilities	695,114	1,602,626
Note Payable	208,506	194,310
Other current liabilities	46,407	46,407
TOTAL CURRENT LIABILITIES	18,701,048	25,643,794
LONG-TERM LIABILITIES
Deferred rent, net of current portion	1,929,052	2,274,997
Deferred revenue, net of current portion		2,500,000
Note Payable, net of current portion	2,325,018	2,533,455
Other non-current liabilities	200,000	200,000
TOTAL LONG-TERM LIABILITIES	4,454,070	7,508,452
Commitments and contingencies (Note 7)
Arrowhead Pharmaceuticals, Inc. stockholders' equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized; 0 and 15,652 shares issued and outstanding as of September 30, 2017 and September 30, 2016		16
Common stock, $0.001 par value; 145,000,000 shares authorized; 74,785,426 and 69,746,685 shares issued and outstanding as of September 30, 2017 and September 30, 2016, respectively	167,155	162,116
Additional paid-in capital	514,037,301	493,844,909
Accumulated other comprehensive income (loss)	33,232	7,449
Accumulated deficit	(432,815,338)	(398,435,043)
Total Arrowhead Pharmaceuticals, Inc. stockholders' equity	81,422,350	95,579,447
Noncontrolling interest	(555,188)	(555,188)
TOTAL STOCKHOLDERS’ EQUITY	80,867,162	95,024,259
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	$ 104,022,280	$ 128,176,505

Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2017	Sep. 30, 2016
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	15,652
Preferred stock, shares outstanding	0	15,652
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	145,000,000	145,000,000
Common stock, shares issued	74,785,426	69,746,685
Common stock, shares outstanding	74,785,426	69,746,685

Consolidated Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
REVENUE	$ 31,407,709	$ 158,333	$ 382,000
OPERATING EXPENSES
Research and development	31,690,298	41,454,452	47,267,361
Acquired in-process research and development			10,142,786
Salaries and payroll-related costs	17,292,883	19,461,656	16,554,008
General and administrative expenses	6,838,402	9,940,737	7,931,184
Stock-based compensation	7,891,595	11,595,816	10,232,897
Depreciation and amortization	4,690,440	3,260,045	2,336,207
Impairment expense		2,050,817
Contingent consideration - fair value adjustments		(5,862,464)	1,891,533
TOTAL OPERATING EXPENSES	68,403,618	81,901,059	96,355,976
OPERATING LOSS	(36,995,909)	(81,742,726)	(95,973,976)
OTHER INCOME (EXPENSE)
Gain (loss) on sale of fixed asset, net			19,195
Interest income (expense), net	415,128	265,794	729,158
Change in value of derivatives	890,362	(301,022)	2,869,267
Other income (expense)	1,312,524	57,352	417,874
TOTAL OTHER INCOME (EXPENSE)	2,618,014	22,124	4,035,494
LOSS BEFORE INCOME TAXES	(34,377,895)	(81,720,602)	(91,938,482)
Provision for income taxes	(2,400)	(2,400)	(2,400)
NET LOSS	$ (34,380,295)	$ (81,723,002)	$ (91,940,882)
NET LOSS PER SHARE - BASIC & DILUTED	$ (0.47)	$ (1.34)	$ (1.60)
Weighted average shares outstanding - basic and diluted	73,898,598	61,050,880	57,358,442
OTHER COMPREHENSIVE INCOME (LOSS), NET OF TAX:
Foreign Currency Translation Adjustments	$ 25,783	$ 143,874	$ (136,425)
COMPREHENSIVE LOSS	$ (34,354,512)	$ (81,579,128)	$ (92,077,307)

Consolidated Statement of Stockholders' Equity - USD ($)	Total	Common stock issued for cash at $5.90 per share	Novartis	Amgen	Preferred Stock	Common Stock	Common Stock Common stock issued for cash at $5.90 per share	Common Stock Novartis	Common Stock Amgen	Additional Paid-In Capital	Additional Paid-In Capital Common stock issued for cash at $5.90 per share	Additional Paid-In Capital Novartis	Additional Paid-In Capital Amgen	Accumulated Other Comprehensive Income (loss)	Accumulated Deficit	Non-controlling Interest
Beginning Balance, Amount at Sep. 30, 2014	$ 165,985,255				$ 18	$ 147,026				$ 391,164,558					$ (224,771,159)	$ (555,188)
Beginning Balance, Shares at Sep. 30, 2014					18,300	54,656,936
Exercise of warrants, Amount	401,876					$ 81				401,795
Exercise of warrants, Shares						79,828
Exercise of stock options, Amount	101,870					$ 29				101,841
Exercise of stock options, Shares						28,758
Stock-based compensation	10,232,897									10,232,897
Exercise of exchange rights, Amount	3,072					$ 5				3,067
Exercise of exchange rights, Shares						5,250
Preferred stock converted to common stock, Amount					$ (2)	$ 1,316				(1,314)
Preferred stock converted to common stock, Shares					(2,648)	1,316,215
Common stock- Restricted Stock Units vesting, Amount	(26,029)					$ 136				(26,165)
Common stock- Restricted Stock Units vesting, Shares						136,307
Stock issuances			$ 25,000,000					$ 3,321				$ 24,996,679
Stock issuances, Shares								3,321,383
Foreign currency translation adjustments	(136,425)													$ (136,425)
Net loss	(91,940,882)														(91,940,882)
Ending Balance, Amount at Sep. 30, 2015	109,621,634				$ 16	$ 151,914				426,873,358				(136,425)	(316,712,041)	(555,188)
Ending Balance, Shares at Sep. 30, 2015					15,652	59,544,677
Exercise of warrants, Amount	3,691,398					$ 853				3,690,545
Exercise of warrants, Shares						852,532
Exercise of stock options, Amount	133,869					$ 37				133,832
Exercise of stock options, Shares						37,187
Stock-based compensation	11,595,816									11,595,816
Common stock- Restricted Stock Units vesting, Amount	(670,765)					$ 428				(671,193)
Common stock- Restricted Stock Units vesting, Shares						428,187
Stock issuances		$ 43,231,435		$ 9,000,000			$ 7,627		$ 1,257		$ 43,223,808		$ 8,998,743
Stock issuances, Shares							7,627,119		1,256,983
Foreign currency translation adjustments	143,874													143,874
Net loss	(81,723,002)														(81,723,002)
Ending Balance, Amount at Sep. 30, 2016	95,024,259				$ 16	$ 162,116				493,844,909				7,449	(398,435,043)	(555,188)
Ending Balance, Shares at Sep. 30, 2016					15,652	69,746,685
Exercise of stock options, Amount	$ 271,931					$ 136				271,795
Exercise of stock options, Shares	135,730					135,730
Stock-based compensation	$ 7,891,595									7,891,595
Exercise of exchange rights, Amount	17,150					$ 5				17,145
Exercise of exchange rights, Shares						5,000
Preferred stock converted to common stock, Amount					$ (16)	$ 2,671				(2,655)
Preferred stock converted to common stock, Shares					(15,652)	2,670,989
Common stock- Restricted Stock Units vesting, Amount	(403,261)					$ 481				(403,742)
Common stock- Restricted Stock Units vesting, Shares						481,212
Stock issuances				$ 12,420,000					$ 1,746				$ 12,418,254
Stock issuances, Shares									1,745,810
Foreign currency translation adjustments	25,783													25,783
Net loss	(34,380,295)														(34,380,295)
Ending Balance, Amount at Sep. 30, 2017	$ 80,867,162					$ 167,155				$ 514,037,301				$ 33,232	$ (432,815,338)	$ (555,188)
Ending Balance, Shares at Sep. 30, 2017						74,785,426

Consolidated Statement of Stockholders' Equity (Parenthetical) - $ / shares	Sep. 30, 2016	Sep. 30, 2015
Common stock issued for cash at $5.90 per share
Stock issued, price per share	$ 5.90
Novartis
Stock issued, price per share		$ 7.53
Amgen
Stock issued, price per share	$ 7.16

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
Consolidated Statements of Cash Flows - USD ($)	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (34,380,295)	$ (81,723,002)	$ (91,940,882)
(Gain) loss on disposal of fixed assets			(19,195)
Change in value of derivatives	(890,362)	301,022	(2,869,267)
Contingent consideration - fair value adjustments		(5,862,464)	1,891,533
Noncash impairment expense		2,050,817
Acquired in-process research and development			10,142,786
Stock-based compensation	7,891,595	11,595,816	10,232,897
Depreciation and amortization	4,690,440	3,260,045	2,336,207
Amortization/(accretion) of note premiums	(43,519)	231,902	1,110,524
Changes in operating assets and liabilities:
Accounts receivable	7,203	(75,000)
Prepaid expenses and Other Current Assets	2,814,509	(1,020,734)	(3,485,421)
Deferred revenue	157,981	5,000,000
Accounts payable	(3,509,995)	2,554,802	2,497,804
Accrued expenses	(401,777)	(871,833)	4,435,784
Other	(274,752)	131,143	(40,385)
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	(23,938,972)	(64,427,486)	(65,707,615)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(7,918,198)	(3,860,237)	(1,970,612)
Proceeds from sale of fixed assets			500
Purchases of marketable securities	(44,974,736)
Proceeds from sale of marketable securities	4,248,716	17,308,000	26,090,950
Cash paid for acquisitions			(10,000,000)
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	(48,644,218)	13,447,763	14,120,838
CASH FLOWS FROM FINANCING ACTIVITIES:
Principal payments on notes payable	(197,790)	(217,549)	(213,991)
Payments of taxes for net share settled restricted stock unit issuances	(438,838)	(634,187)
Proceeds from the exercises of warrants and stock options	272,818	3,752,120	504,512
Proceeds from the issuance of common stock	12,419,119	52,231,433
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	12,055,309	55,131,817	290,521
NET INCREASE (DECREASE) IN CASH	(60,527,881)	4,152,094	(51,296,256)
CASH AT BEGINNING OF PERIOD	85,366,448	81,214,354	132,510,610
CASH AT END OF PERIOD	24,838,567	85,366,448	81,214,354
Supplementary disclosures:
Interest Paid	(187,647)	(11,287)	(14,429)
Property and equipment purchased through tenant improvement allowance financing		(4,849,360)
Property and Equipment expenditures included in accounts payable and accrued expenses		(4,801,930)
Income Tax Credits Refunded	3,635,016	1,365,288
Income Tax Paid	$ (2,400)	$ (2,400)	(2,400)
Novartis
Supplementary disclosures:
Common Stock issued to Novartis for asset acquisition			$ (25,000,000)

Organization and Significant Accounting Policies	12 Months Ended
Organization and Significant Accounting Policies	Sep. 30, 2017
Accounting Policies [Abstract]
Organization and Significant Accounting Policies	NOTE 1. ORGANIZATION AND SIGNIFICANT ACCOUNTING POLICIES Nature of Business and Recent Developments Arrowhead Pharmaceuticals, Inc. develops medicines that treat intractable diseases by silencing the genes that cause them. Using a broad portfolio of RNA chemistries and efficient modes of delivery, Arrowhead therapies trigger the RNA interference mechanism to induce rapid, deep and durable knockdown of target genes. RNA interference, or RNAi, is a mechanism present in living cells that inhibits the expression of a specific gene, thereby affecting the production of a specific protein. Deemed to be one of the most important recent discoveries in life science with the potential to transform medicine, the discoverers of RNAi were awarded a Nobel Prize in 2006 for their work. Arrowhead’s RNAi-based therapeutics leverage this natural pathway of gene silencing. The company's pipeline includes ARO-HBV for chronic hepatitis B virus, ARO-AAT for liver disease associated with alpha-1 antitrypsin deficiency (AATD), ARO-APOC3 and ARO-ANG3 for hypertriglyceridemia, ARO-Lung1 for an undisclosed pulmonary target, ARO-HIF2 for renal cell carcinoma, ARO-F12 for hereditary angioedema and thromboembolic disorders, and ARO-AMG1 for an undisclosed genetically validated cardiovascular target under a license and collaboration agreement with Amgen, Inc., a Delaware corporation (“Amgen”). ARO-LPA (AMG 890) for cardiovascular disease was out-licensed to Amgen in 2016. Arrowhead operates a lab facility in Madison, Wisconsin, where the Company’s research and development activities, including the development of RNAi therapeutics, are based. The Company’s principal executive offices are located in Pasadena, California. In fiscal 2017, Arrowhead refocused its resources on therapeutics that exclusively utilize the company’s Targeted RNAi Molecule (TRiM^TM) platform technology. Therapeutics built on the TRiM^TM platform have demonstrated high levels of pharmacologic activity in multiple animal models spanning several therapeutic areas. TRiM^TM enabled therapeutics offer several potential advantages over prior generation and competing technologies, including: simplified manufacturing and reduced costs; multiple routes of administration including subcutaneous injection and inhaled administration; the ability to target multiple tissue types including liver, lung, and tumors; and the potential for improved safety and reduced risk of intracellular buildup, because there are less metabolites from smaller, simpler molecules. As part of the refocusing of resources, Arrowhead announced in November 2016 that it would be discontinuing all clinical programs that utilized the intravenously administered DPC, or EX1, delivery vehicle. The decision to discontinue development of EX1-containing programs was based primarily on two factors. First, during discussions with regulatory agencies and outside experts, it became apparent that there would be substantial delays in all clinical programs that utilize EX1, while the Company further explored the cause of deaths in a non-clinical toxicology study in non-human primates exploring doses of EX1 higher than those planned to be used in humans. Second, the Company had made substantial advances in RNA chemistry and targeting resulting in large potency gains for development programs utilizing the TRiM^TM technology, making EX1 no longer necessary. Liquidity The Consolidated Financial Statements have been prepared in conformity with the accounting principles generally accepted in the United States of America, which contemplate the continuation of the Company as a going concern. Historically, the Company’s primary source of financing has been through the sale of its securities. Research and development activities have required significant capital investment since the Company’s inception. The Company expects its operations to continue to require cash investment to pursue its research and development goals, including clinical trials and related drug manufacturing. At September 30, 2017, the Company had $24.8 million in cash to fund operations. In addition to its cash resources, the Company has invested excess cash in investment grade commercial bonds maturing in less than 12 months. At September 30, 2017, the Company had invested $40.8 million in bonds. During the year ended September 30, 2017, the Company’s cash and investments balances combined decreased by $19.8 million, which was the result of $53.9 million in cash outflows related to operating activities (excluding the upfront payment from Amgen discussed below), and $7.9 million of capital expenditures, offset by $42.5 million in upfront payments and equity investments from Amgen. On November 18, 2016, the Company and Amgen received Hart-Scott-Rodino clearance with regard to the ARO-LPA (AMG 890) Agreement discussed in Note 2 below. Based on the terms of this agreement, and the Common Stock Purchase Agreement, the Company issued 1,745,810 shares of Common Stock to Amgen, and received proceeds of approximately $12.5 million. Additionally, the Company received a $30 million upfront payment due under the ARO-LPA (AMG 890) Agreement discussed below. Summary of Significant Accounting Policies Principles of Consolidation—The consolidated financial statements include the accounts of Arrowhead and its Subsidiaries. Arrowhead’s primary operating subsidiary is Arrowhead Madison, which is located in Madison, Wisconsin, where the Company’s research and development facility is located. All significant intercompany accounts and transactions are eliminated in consolidation. Basis of Presentation and Use of Estimates—The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could materially differ from those estimates. Additionally, certain reclassifications have been made to prior period financial statements to conform to the current period presentation. Cash and Cash Equivalents—The Company considers all liquid debt instruments purchased with a maturity of three months or less to be cash equivalents. The Company had no restricted cash at September 30, 2017 and September 30, 2016. Concentration of Credit Risk—The Company maintains several bank accounts at two financial institutions for its operations. These accounts are insured by the Federal Deposit Insurance Corporation (FDIC) for up to $250,000 per institution. Management believes the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which these deposits are held. Investments—The Company may invest excess cash balances in short-term and long-term marketable debt securities. Investments may include certificates of deposits, money market accounts, government-sponsored enterprise securities, corporate bonds and/or commercial paper. The Company accounts for its investment in marketable securities in accordance with FASB ASC 320, Investments – Debt and Equity Securities. This statement requires certain securities to be classified into three categories: Held-to-maturity—Debt securities that the entity has the positive intent and ability to hold to maturity are reported at amortized cost. Trading Securities—Debt and equity securities that are bought and held primarily for the purpose of selling in the near term are reported at fair value, with unrealized gains and losses included in earnings. Available-for-Sale—Debt and equity securities not classified as either securities held-to-maturity or trading securities are reported at fair value with unrealized gains or losses excluded from earnings and reported as a separate component of shareholders’ equity. The Company classifies its investments in marketable debt securities based on the facts and circumstances present at the time of purchase of the securities. During the year ended September 30, 2017, all of the Company’s investments were classified as held-to-maturity. Held-to-maturity investments are measured and recorded at amortized cost on the Company’s Consolidated Balance Sheet. Discounts and premiums to par value of the debt securities are amortized to interest income/expense over the term of the security. No gains or losses on investment securities are realized until they are sold or a decline in fair value is determined to be other-than-temporary. Property and Equipment—Property and equipment are recorded at cost, which may equal fair market value in the case of property and equipment acquired in conjunction with a business acquisition. Depreciation of property and equipment is recorded using the straight-line method over the respective useful lives of the assets ranging from three to seven years. Leasehold improvements are amortized over the lesser of the expected useful life or the remaining lease term. Long-lived assets, including property and equipment are reviewed for impairment whenever events or circumstances indicate that the carrying amount of these assets may not be recoverable. Intangible Assets Subject to Amortization—Intangible assets subject to amortization include certain patents and a license agreement. Intangible assets subject to amortization are reviewed for impairment whenever events or circumstances indicate that the carrying amount of these assets may not be recoverable. Contingent Consideration - The consideration for the Company’s acquisitions may include potential future payments that are contingent upon the occurrence of a particular event. For example, milestone payments might be based on the achievement of various regulatory approvals or future sales milestones, and royalty payments might be based on drug product sales levels. The Company records a contingent consideration obligation for such contingent payments at fair value on the acquisition date. The Company estimates the fair value of contingent consideration obligations through valuation models designed to estimate the probability of such contingent payments based on various assumptions and incorporating estimated success rates. Estimated payments are discounted using present value techniques to arrive at an estimated fair value at the balance sheet date. Changes in the fair value of the contingent consideration obligations are recognized within the Company’s Consolidated Statements of Operations and Comprehensive Loss. Changes in the fair value of the contingent consideration obligations can result from changes to one or multiple inputs, including adjustments to the discount rates, changes in the amount or timing of expected expenditures associated with product development, changes in the amount or timing of cash flows from products upon commercialization, changes in the assumed achievement or timing of any development milestones, changes in the probability of certain clinical events and changes in the assumed probability associated with regulatory approval. These fair value measurements are based on significant inputs not observable in the market. Substantial judgment is employed in determining the appropriateness of these assumptions as of the acquisition date and for each subsequent period. Accordingly, changes in assumptions could have a material impact on the amount of contingent consideration expense the Company records in any given period. The Company determined the fair value of its contingent consideration obligation to be $0 at September 30, 2017 and September 30, 2016, given the discontinuation of its clinical trials. Revenue Recognition— Revenue from product sales is recorded when persuasive evidence of an arrangement exists, title has passed and delivery has occurred, a price is fixed and determinable, and collection is reasonably assured. The Company may generate revenue from technology licenses, collaborative research and development arrangements, and product sales. Revenue under technology licenses and collaborative agreements typically consists of nonrefundable and/or guaranteed technology license fees, collaborative research funding, manufacturing and development services and various milestone and future product royalty or profit-sharing payments. These agreements are considered to be multiple element arrangements. The Company applies the accounting standard on revenue recognition for multiple element arrangements. The fair value of deliverables under the arrangement may be derived using a best estimate of selling price if vendor specific objective evidence and third-party evidence is not available. Deliverables under the arrangement will be separate units of accounting if a delivered item has value to the customer on a standalone basis, if the arrangement includes a general right of return for the delivered item, and if delivery or performance of the undelivered item is considered probable and substantially in the Company’s control. The Company recognizes upfront license payments as revenue upon delivery of the license only if the license has standalone value from any undelivered performance obligations and that value can be determined. The undelivered performance obligations typically include manufacturing or development services or research and/or steering committee services. If the fair value of the undelivered performance obligations can be determined, then these obligations would be accounted for separately. If the license is not considered to have standalone value, then the license and other undelivered performance obligations would be accounted for as a single unit of accounting. In this case, the license payments and payments for performance obligations are recognized as revenue over the estimated period of when the performance obligations are performed or deferred indefinitely until the undelivered performance obligation is determined. Whenever the Company determines that an arrangement should be accounted for as a single unit of accounting, the Company determines the period over which the performance obligations will be performed and revenue will be recognized. Revenue is recognized using a proportional performance or straight-line method. The proportional performance method is used when the level of effort required to complete performance obligations under an arrangement can be reasonably estimated. The amount of revenue recognized under the proportional performance method is determined by multiplying the total payments under the contract, excluding royalties and payments contingent upon achievement of milestones, by the ratio of the level of effort performed to date to the estimated total level of effort required to complete performance obligations under the arrangement. If the Company cannot reasonably estimate the level of effort to complete performance obligations under an arrangement, the Company recognizes revenue under the arrangement on a straight-line basis over the period the Company is expected to complete its performance obligations. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company is expected to complete its performance obligations under an arrangement. Many of the Company’s collaboration agreements entitle the Company to additional payments upon the achievement of development, regulatory and sales performance-based milestones. If the achievement of a milestone is considered probable at the inception of the collaboration agreement, the related milestone payment is included with other collaboration consideration, such as upfront fees and research funding, in the Company’s revenue calculation. Typically these milestones are not considered probable at the inception of the collaboration. As such, milestones will typically be recognized in one of two ways depending on the timing of when the milestone is achieved. If the milestone is achieved during the performance period, the Company will only recognize revenue to the extent of the proportional performance achieved at that date, or the proportion of the straight-line basis achieved at that date, and the remainder will be recorded as deferred revenue to be amortized over the remaining performance period. If the milestone is achieved after the performance period has completed and all performance obligations have been delivered, the Company will recognize the milestone payment as revenue in its entirety in the period the milestone was achieved. Deferred revenue will be classified as part of Current or Long-Term Liabilities in the accompanying Consolidated Balance Sheets based on the Company’s estimate of the portion of the performance obligations regarding that revenue will be completed within the next 12 months divided by the total performance period estimate. This estimate is based on the Company’s current operating plan and, if the Company’s operating plan should change in the future, the Company may recognize a different amount of deferred revenue over the next 12-month period. Allowance for Doubtful Accounts—The Company accrues an allowance for doubtful accounts based on estimates of uncollectible revenues by analyzing historical collections, accounts receivable aging and other factors. Accounts receivable are written off when all collection attempts have failed. Research and Development—Costs and expenses that can be clearly identified as research and development are charged to expense as incurred in accordance with FASB ASC 730-10. Included in research and development costs are operating costs, facilities, supplies, external services, clinical trial and manufacturing costs, overhead directly related to the Company’s research and development operations, and costs to acquire technology licenses. Earnings (Loss) per Share—Basic earnings (loss) per share is computed using the weighted-average number of common shares outstanding during the period. Diluted earnings (loss) per share are computed using the weighted-average number of common shares and dilutive potential common shares outstanding during the period. Dilutive potential common shares primarily consist of stock options and restricted stock units issued to employees and warrants to purchase Common Stock of the Company. All outstanding stock options, restricted stock units and warrants for the years ended September 30, 2017, 2016 and 2015 have been excluded from the calculation of Diluted earnings (loss) per share due to their anti-dilutive effect. Stock-Based Compensation—The Company accounts for share-based compensation arrangements in accordance with FASB ASC 718, which requires the measurement and recognition of compensation expense for all share-based payment awards to be based on estimated fair values. The Company uses the Black-Scholes option valuation model to estimate the fair value of its stock options at the date of grant. The Black-Scholes option valuation model requires the input of subjective assumptions to calculate the value of stock options. For restricted stock units, the value of the award is based on the Company’s stock price at the grant date. For performance-based restricted stock unit awards, the value of the award is based on the Company’s stock price at the grant date. The Company uses historical data and other information to estimate the expected price volatility for stock option awards and the expected forfeiture rate for all awards. Expense is recognized over the vesting period for all awards, and commences at the grant date for time-based awards and upon the Company’s determination that the achievement of such performance conditions is probable for performance-based awards. This determination requires significant judgment by management. Derivative Assets and Liabilities – The Company accounts for warrants and other derivative financial instruments as either equity or assets/liabilities based upon the characteristics and provisions of each instrument. Warrants classified as equity are recorded as Additional Paid-In Capital on the Company’s Consolidated Balance Sheet. Some of the Company’s warrants were determined to be ineligible for equity classification due to provisions that may result in an adjustment to their exercise price. Warrants classified as derivative liabilities and other derivative financial instruments that require separate accounting as assets or liabilities are recorded on the Company’s Consolidated Balance Sheet at their fair value on the date of issuance and are revalued on each subsequent balance sheet date until such instruments are exercised or expire, with any changes in the fair value between reporting periods recorded as Other Income or Expense. The Company estimates the fair value of these assets/liabilities using option pricing models that are based on the individual characteristics of the warrants or instruments on the valuation date, as well as assumptions for expected volatility, expected life and risk-free interest rate. Income Taxes—The Company accounts for income taxes under the liability method, which requires the recognition of deferred income tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Under this method, deferred income taxes are recognized for the tax consequences in future years of differences between the tax bases of assets and liabilities and their financial reporting amounts at each period end based on enacted tax laws and statutory tax rates applicable to the periods in which the differences are expected to affect taxable income. Valuation allowances are established, when necessary, to reduce deferred income tax assets to the amount expected to be realized. The provision for income taxes, if any, represents the tax payable for the period and the change in deferred income tax assets and liabilities during the period. Recent Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09 Revenue from Contracts with Customers (Topic 606), which will supersede nearly all existing revenue recognition guidance under GAAP. ASU No. 2014-09 provides that an entity recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. This update also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments, and assets recognized from costs incurred to obtain or fulfill a contract. ASU No. 2014-09 allows for either full retrospective or modified retrospective adoption and will become effective for the Company in the first quarter of 2019. The Company is evaluating the potential effects of the adoption of this update on its financial statements. In March 2016, the FASB issued ASU No. 2016-02, Leases. Under ASU 2016-02, lessees will be required to recognize a right-of-use asset and a lease liability for virtually all of their leases (other than leases that meet the definition of a short-term lease). For income statement purposes, a dual model was retained, requiring leases to be classified as either operating or finance. Operating leases will result in straight-line expense (similar to current operating leases) while finance leases will result in a front-loaded expense pattern (similar to current capital leases). ASU 2016-02 becomes effective for the Company in the first quarter of fiscal 2020. The Company expects the adoption of this update to have a material effect on the classification and disclosure of its leased facilities in Madison, Wisconsin. In May 2017, the FASB issued ASU No. 2017-09, which is an update to Topic 718, Compensation - Stock Compensation. The update provides guidance on determining which changes to the terms and conditions of share-based payment awards, including stock options, require an entity to apply modification accounting under Topic 718. ASU 2017-09 becomes effective for the Company in the first quarter of fiscal 2019. The Company does not expect that ASU 2017-09 will have a material impact on the Company's results of operations and consolidated financial statements.

Collaboration and License Agreements – Amgen, Inc	12 Months Ended
Collaboration and License Agreements – Amgen, Inc	Sep. 30, 2017
Collaboration And License Agreements [Abstract]
Collaboration and License Agreements – Amgen, Inc	NOTE 2. COLLABORATION AND LICENSE AGREEMENTS – AMGEN, INC. On September 28, 2016, the Company entered into two Collaboration and License agreements, and a Common Stock Purchase Agreement with Amgen Inc., a Delaware corporation (“Amgen”). Under one of the license agreements (the “Second Collaboration and License Agreement” or “ARO-LPA (AMG 890)Agreement”), Amgen has received a worldwide, exclusive license to Arrowhead’s novel, RNAi ARO-LPA (AMG 890)program. These RNAi molecules are designed to reduce elevated lipoprotein(a), which is a genetically validated, independent risk factor for atherosclerotic cardiovascular disease. Under the other license agreement (the “First Collaboration and License Agreement” or “ARO-AMG1 Agreement”), Amgen received an option to a worldwide, exclusive license for ARO-AMG1, an RNAi therapy for an undisclosed genetically validated cardiovascular target. In both agreements, Amgen is wholly responsible for clinical development and commercialization. Under the Common Stock Purchase Agreement, the Company has sold 3,002,793 shares of Common Stock to Amgen at a price of $7.16 per share, which represents the 30-day volume-weighted average price of the Common Stock on the NASDAQ stock market over the 30 trading days preceding contract execution. Subject to Amgen’s exercise of the Option, as defined in the ARO-AMG1 Agreement, Amgen has agreed to purchase, and the Company has agreed to sell, an additional $5 million worth of shares of Common Stock based on a 30 trading day formula surrounding the date of the Option exercise. Under the terms of the agreements taken together, the Company has received $35 million in upfront payments, $21.5 million in the form of an equity investment by Amgen in the Company’s Common Stock, and could receive up to $617 million in option payments, and development, regulatory and sales milestone payments. The Company is further eligible to receive single-digit royalties for sales of products under the ARO-AMG1 Agreement and up to low double-digit royalties for sales of products under the ARO-LPA (AMG 890)Agreement. Under the terms of the ARO-AMG1 Agreement, the Company has granted an option to a worldwide, exclusive license to ARO-AMG1, an undisclosed genetically validated cardiovascular target. The collaboration between the Company and Amgen is governed by a joint steering committee comprised of an equal number of representatives from each party. The Company is also responsible for developing, optimizing and manufacturing the candidate through certain preclinical efficacy and toxicology studies to determine whether the candidate the Company has developed meets the required criteria as defined in the agreement (the “Arrowhead Deliverable”). If this is achieved, Amgen will then have the option to an exclusive license for the intellectual property generated through the Company’s development efforts, and will likely assume all development, regulatory and commercialization efforts for the candidate upon the option exercise. The Company has determined that the significant deliverables under the ARO-AMG1 Agreement include the license, the joint research committee and the development and manufacturing activities toward achieving the Arrowhead Deliverable. The Company also determined that, pursuant to the accounting guidance governing revenue recognition on multiple element arrangements, the license and collective undelivered activities and services do not have standalone value due to the specialized nature of the activities and services to be provided by the Company. Therefore, the deliverables are not separable and, accordingly, the license and undelivered services are being treated as a single unit of accounting. The Company will recognize revenue on a straight-line basis from October 1, 2016, through September 30, 2018. The due date for achieving the Arrowhead Deliverable is September 28, 2018. The initial $5 million upfront payment was recorded as Deferred Revenue and $2.5 million of this initial payment was amortized into Revenue during the year ended September 30, 2017. Should the Company achieve the Arrowhead Deliverable prior to the due date, unrecognized revenue will be accelerated and recognized. Under the terms of the ARO-LPA (AMG 890)Agreement, the Company has granted a worldwide, exclusive license to ARO-LPA (AMG 890). The collaboration between the Company and Amgen is governed by a joint research committee comprised of an equal number of representatives from each party, however Amgen has the final decision making authority regarding ARO-LPA (AMG 890)in this committee. The Company is also responsible for assisting Amgen in the oversight of certain development and manufacturing activities, most of which are to be covered at Amgen’s cost. The Company has determined that the significant deliverables under the ARO-LPA (AMG 890) Agreement include the license and the oversight of certain of the development and manufacturing activities. The Company also determined that, pursuant to the accounting guidance governing revenue recognition on multiple element arrangements, the license and collective undelivered activities and services do not have standalone value due to the specialized nature of the activities and services to be provided by the Company. Therefore, the deliverables are not separable and, accordingly, the license and undelivered services are being treated as a single unit of accounting. The Company will recognize revenue on a straight-line basis from November 18, 2016 (the Hart-Scott-Rodino clearance date), through October 31, 2017, which is the date where the significant development and manufacturing related deliverables were completed. The initial $30 million payment was recorded as Deferred Revenue, and $27.3 million of this initial payment was amortized into Revenue during the year ended September 30, 2017. In January 2017, the Company also entered into a separate services agreement with Amgen to provide certain services related to process development, manufacturing, materials supply, discovery studies, and other consulting services related to ARO-LPA (AMG 890). During the year ended September 30, 2017, work orders under this services agreement generated approximately $1.5 million of Revenue.

Property and Equipment

12 Months Ended

Sep. 30, 2017

Property Plant And Equipment [Abstract]

Property and Equipment

NOTE 3. PROPERTY AND EQUIPMENT

The following table summarizes the Company’s major classes of property and equipment:


	September 30, 2017			September 30, 2016
Computers, office equipment and furniture	$	600,334		$	442,915
Research equipment		9,660,960			7,490,400
Software		132,078			80,841
Leasehold improvements		12,208,380			11,885,365
Total gross fixed assets		22,601,752			19,899,521
Less: Accumulated depreciation and amortization		(7,088,733	)		(4,512,760	)
Property and equipment, net	$	15,513,019		$	15,386,761

During the year ended September 30, 2017, the Company’s research equipment increased as the Company has moved into a larger research facility in Madison, Wisconsin.

Investments

12 Months Ended

Sep. 30, 2017

Investments All Other Investments [Abstract]

Investments

NOTE 4. INVESTMENTS

The Company invests a portion of its excess cash balances in short-term debt securities and may, from time to time, also invest in long-term debt securities. Investments at September 30, 2017 consisted of corporate bonds with maturities remaining of less than one year. The Company may also invest excess cash balances in certificates of deposits, money market accounts, government-sponsored enterprise securities, corporate bonds and/or commercial paper. The Company accounts for its investments in accordance with FASB ASC 320, Investments – Debt and Equity Securities. At September 30, 2017, all investments were classified as held-to-maturity securities.

The following tables summarize the Company’s short-term investments as of September 30, 2017, and September 30, 2016.

	As of September 30, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Commercial notes (due within one year)	$	40,769,539	$	—	$	(334,755)	$	40,434,784

As of September 30, 2016

Amortized Cost

Gross Unrealized Gains

Gross Unrealized Losses

Fair Value

Commercial notes (due within one year)

—

Intangible Assets

12 Months Ended

Sep. 30, 2017

Goodwill And Intangible Assets Disclosure [Abstract]

Intangible Assets

NOTE 5. INTANGIBLE ASSETS

Intangible assets subject to amortization include patents and a license agreement capitalized as part of the Novartis RNAi asset acquisition in March 2015. The license agreement associated with the Novartis RNAi asset acquisition is being amortized over the estimated life remaining at the time of acquisition, which was 21 years, and the accumulated amortization of the asset is approximately $383,330. The patents associated with the Novartis RNAi asset acquisition are being amortized over the estimated life remaining at the time of acquisition, which was 14 years, and the accumulated amortization of the assets is approximately $4,009,395. Amortization expense for the years ended September 30, 2017, 2016 and 2015 was $1,700,429, $1,714,313 and $1,046,571, respectively. Amortization expense is expected to be approximately $1,700,429 for fiscal year 2018, $1,700,429 in 2019, $1,700,429 in 2020, $1,700,429 in 2021, $1,700,429 in 2022, and $11,962,294 thereafter.

The following table provides details on the Company’s intangible asset balances:

	Intangible assets subject to amortization
Balance at September 30, 2016		$	22,164,868
Impairment			-
Amortization			(1,700,429	)
Balance at September 30, 2017		$	20,464,439

Stockholders' Equity

12 Months Ended

Sep. 30, 2017

Equity [Abstract]

Stockholders' Equity

NOTE 6. STOCKHOLDERS’ EQUITY

At September 30, 2017, the Company had a total of 150,000,000 shares of capital stock authorized for issuance, consisting of 145,000,000 shares of Common Stock, par value $0.001 per share, and 5,000,000 shares of Preferred Stock, par value $0.001 per share.

At September 30, 2017, 74,785,426 shares of Common Stock were outstanding. At September 30, 2017, 8,902,416 shares of Common Stock were reserved for issuance upon exercise of options and vesting of restricted stock units granted or available for grant under Arrowhead’s 2004 Equity Incentive Plan and 2013 Incentive Plan, as well as for inducement grants made to new employees.

During the year ended September 30, 2017, 15,652 shares of Series C Preferred Stock were converted into 2,670,989 shares of Common Stock. No preferred stock was outstanding as of September 30, 2017.

On March 21, 2017, the Board of Directors (the “Board”) of the Company authorized and declared a dividend distribution of one right (a “Right”) for each outstanding share of Common Stock of the Company to stockholders of record at the close of business on March 22, 2017 (the “Record Date”). Each Right entitles the registered holder to purchase from the Company one one-thousandth of a share of Series D Junior Participating Preferred Stock, par value $0.001 per share (the “Preferred Shares”), of the Company at a purchase price of $20 per one one-thousandth of a Preferred Share (the “Purchase Price”). This portion of a share of Preferred Stock would give the holder thereof approximately the same dividend, voting and liquidation rights as would one share of Common Stock, with any variations set forth in the Certificate of Designation, Preferences, and Rights of Series D Junior Participating Preferred Stock attached hereto as Exhibit 3.3. The Rights become exercisable on the earlier of (i) 10 business days following a public announcement that a person has become an “Acquiring Person” by acquiring beneficial ownership of 15% or more of the Common Stock then outstanding, other than as a result of repurchases of Common Stock by the Company or certain inadvertent acquisitions; or (ii) 10 business days (or such later date as the Board shall determine prior to the time a person becomes an Acquiring Person) after the commencement of a tender offer or an exchange offer by or on behalf of any person (other than the Company and certain related entities) that, if completed, would result in such person becoming an Acquiring Person. In the event that a person becomes an Acquiring Person, each holder of a Right shall thereafter have the right to receive, upon exercise, Common Stock (or, in certain circumstances, other securities, cash, or other assets of the Company) having a value equal to two times the Purchase Price. The Rights expire on March 21, 2018.

On September 28, 2016, the Company sold 1,256,983 shares of Common Stock to Amgen, Inc. at a price of $7.16 per share as part of the Common Stock Purchase Agreement executed with Amgen and discussed further in Note 2 – Collaboration and License Agreements – Amgen, Inc. The Company received proceeds of $9 million. On November 18, 2016, a second tranche of 1,745,810 shares was also sold to Amgen at a price of $7.16 per share as part of the Common Stock Purchase Agreement. The Company received proceeds of $12.5 million in November 2016.

The following table summarizes information about warrants outstanding at September 30, 2017:

Exercise prices		Number of Warrants		Remaining Life in Years
$	2.12		75,000		0.4
$	1.83		277,284		0.2
$	7.14		80,000		0.7
Total warrants outstanding			432,284

Commitments and Contingencies

12 Months Ended

Sep. 30, 2017

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

NOTE 7. COMMITMENTS AND CONTINGENCIES

Leases

The Company leases approximately 8,500 square feet of office space for its corporate headquarters in Pasadena, California. The lease will expire in September 2019. Rental costs, including common area maintenance and real estate taxes, are approximately $27,000 per month, increasing approximately 3% annually.

The Company also leases approximately 60,000 square feet of office and laboratory space for its research facility in Madison, Wisconsin. The lease will expire in September 2026. As part of this lease, the Company was provided a primary tenant improvement allowance of $2.1 million, which is accounted for as Deferred Rent and a secondary tenant improvement allowance of $2.7 million, which is accounted for as a Note Payable on the Company’s Consolidated Balance Sheet. Monthly rental payments, including common area maintenance, real estate taxes and payments of principal and interest on the Note Payable are approximately $182,200 per month. The monthly rental payments (excluding principal and interest on the Note Payable), will increase approximately 2.5% annually.

The Company previously leased additional research facility space in Middleton, Wisconsin; however this lease expired in December 2016. Monthly rental payments including common area maintenance and real estate taxes for the additional space was approximately $18,000.

Facility rent expense for the years ended September 30, 2017, 2016 and 2015 was $1,554,000, $926,000 and $831,000, respectively.

As of September 30, 2017, future minimum lease payments due in fiscal years under operating leases are as follows:

2018	$	1,531,234
2019		1,435,409
2020		1,044,431
2021		1,070,496
2022		1,097,168
2023 and thereafter		4,669,328
Total	$	10,848,066

Note Payable

As part of the Company’s lease for its research facility in Madison, Wisconsin discussed above, the Company entered into a $2.7 million promissory note payable with its landlord to finance certain tenant improvements made to the new facility. The note will be amortized over the 10-year term of the lease, commencing on October 1, 2016. The note will bear interest at a rate of 7.1% and shall be payable in equal monthly installments of principal and interest.

As of September 30, 2017, future principal payments due in fiscal years under the note payable are as follows:

2018	$	208,506
2019		223,820
2020		240,258
2021		257,903
2022		276,845
2023 and thereafter		1,326,192
Total	$	2,533,524

Litigation

The Company and certain of its officers and directors were named as defendants in a putative consolidated class action in the United States District Court for the Central District of California regarding certain public statements in connection with the Company’s hepatitis B drug research. The consolidated class action, initially filed as Wang v. Arrowhead Research Corp., et al., No. 2:14-cv-07890 (C.D. Cal., filed Oct. 10, 2014), and Eskinazi v. Arrowhead Research Corp., et al., No. 2:14-cv-07911 (C.D. Cal., filed Oct. 13, 2014), asserted claims under Sections 10(b) and 20(a) of the Securities Exchange Act of 1934 and sought damages in an unspecified amount. Additionally, three putative stockholder derivative actions captioned Weisman v. Anzalone et al., No. 2:14-cv-08982 (C.D. Cal., filed Nov. 20, 2014), Bernstein (Backus) v. Anzalone, et al., No. 2:14-cv-09247 (C.D. Cal., filed Dec. 2, 2014); and Johnson v. Anzalone, et al., No. 2:15-cv-00446 (C.D. Cal., filed Jan. 22, 2015), were filed in the United States District Court for the Central District of California, alleging breach of fiduciary duty by the Company’s Board of Directors in connection with the alleged facts underlying the securities claims. An additional consolidated derivative action asserting similar claims is pending in Los Angeles County Superior Court, initially filed as Bacchus v. Anzalone, et al., (L.A. Super., filed Mar. 5, 2015); and Jackson v. Anzalone, et al. (L.A. Super., filed Mar. 16, 2015). Each of these suits seeks damages in unspecified amounts and some seek various forms of injunctive relief. On October 7, 2016, the federal district court dismissed the consolidated class action with prejudice. On October 10, 2016, the plaintiffs appealed the dismissal of the consolidated class action to the United States Court of Appeals for the Ninth Circuit. The Weisman and Johnson derivative actions have been dismissed without prejudice. The Bernstein derivative action remains pending and is stayed pending the related consolidated class action. The Company believes it has meritorious defenses and intends to vigorously defend itself in each of these matters. The Company makes provisions for liabilities when it is both probable that a liability has been incurred and the amount can be reasonably estimated. No such liability has been recorded related to these matters. The Company does not expect these matters to have a material effect on its Consolidated Financial Statements. With regard to legal fees, such as attorney fees related to these matters or any other legal matters, the Company recognizes such costs as incurred.

The Company and certain executive officers were named as defendants in a putative consolidated class action in the United States District Court for the Central District of California regarding certain public statements in connection with the Company’s drug research programs. The consolidated class action, initially filed as Meller v. Arrowhead Pharmaceuticals, Inc., et al., No. 2:16-cv-08505 (C.D. Cal, filed Nov. 15, 2016 ), Siegel v. Arrowhead Pharmaceuticals, Inc., et al., No. 2:16-cv-8954 (C.D. Cal., filed Dec. 2, 2016), and Unz v. Arrowhead Pharmaceuticals, Inc., et al., No.2:17-cv-00310 (C.D. Cal., filed Jan. 13, 2017) asserts claims under Sections 10(b) and 20(a) of the Securities Exchange Act of 1934 regarding certain public statements in connection with the Company’s drug research programs and seek damages in an unspecified amount. Additionally, a putative stockholder derivative action captioned Johnson v. Anzalone, et al., (Los Angeles County Superior Court, filed January 19, 2017) asserting substantially similar claims is pending in Los Angeles County Superior Court and is stayed pending the related consolidated class action. Two additional putative stockholder derivative actions, captioned Lucas v. Anzalone, et al., No. 2:17-cv-03207 (C.D. Cal., filed April 28, 2017), and Singh v. Anzalone, et al., No. 2:17-cv-03160 (C.D. Cal., filed April 27, 2017), alleging breach of fiduciary duty by the Company’s Board of Directors in connection with the alleged facts underlying the securities claims, are pending in the United States District Court for the Central District of California. The Lucas and Singh actions have been consolidated. On September 20, 2017, the United States District Court dismissed the consolidated class action without prejudice. On October 26, 2017, the plaintiffs filed a second amended consolidated complaint, and a motion to dismiss was filed on November 8, 2017. The Company believes it has meritorious defenses and intends to vigorously defend itself in these matters. The Company makes provisions for liabilities when it is both probable that a liability has been incurred and the amount can be reasonably estimated. No such liability has been recorded related to these matters. The Company cannot predict the ultimate outcome of this matter and cannot accurately estimate any potential liability the Company may incur or the impact of the results of this matter on the Company. With regard to legal fees, such as attorney fees related to these matters or any other legal matters, the Company recognizes such costs as incurred.

Purchase Commitments

In the normal course of business, we enter into various purchase commitments for the manufacture of drug components, for toxicology studies, and for pre-clinical and clinical studies. As of September 30, 2017, these future commitments were estimated at approximately $11.4 million, all of which are expected to be incurred in fiscal 2018.

Technology License Commitments

The Company has licensed from third parties the rights to use certain technologies for its research and development activities, as well as in any products the Company may develop using these licensed technologies. These agreements and other similar agreements often require milestone and royalty payments. Milestone payments, for example, may be required as the research and development process progresses through various stages of development, such as when clinical candidates enter or progress through clinical trials, upon NDA and upon certain sales level milestones. These milestone payments could amount to the mid to upper double-digit millions of dollars. During the year ended September 30, 2017, 2016, and 2015, we reached milestones amounting to $0, $3.0 million and $1.0 million, respectively, based on progress achieved on our previous clinical candidates. In certain agreements, the Company may be required to make mid to high single-digit percentage royalty payments based on a percentage of the sales of the relevant products.

Stock-Based Compensation

12 Months Ended

Sep. 30, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

NOTE 8. STOCK-BASED COMPENSATION

Arrowhead has two plans that provide for equity-based compensation. Under the 2004 Equity Incentive Plan and 2013 Incentive Plan, as of September 30, 2017, 2,119,286 and 6,329,079 shares, respectively, of Arrowhead’s Common Stock are reserved for the grant of stock options, stock appreciation rights, restricted stock awards and performance unit/share award to employees, consultants and others. No further grants may be made under the 2004 Equity Incentive Plan. As of September 30, 2017, there were options granted and outstanding to purchase 2,119,286 and 2,986,207 shares of Common Stock under the 2004 Equity Incentive Plan and the 2013 Incentive Plan, respectively, and there were 3,098,000 restricted stock units granted and outstanding under the 2013 Incentive Plan. Also, as of September 30, 2017, there were 444,050 shares reserved for options and 10,000 restricted stock units issued as inducement grants to new employees outside of equity compensation plans. During the year ended September 30, 2017, no options or restricted stock units were granted under the 2004 Equity Incentive Plan, 591,000 options and 2,623,000 restricted stock units were granted under the 2013 Incentive Plan, and 47,000 options and 0 restricted stock units were granted as inducement awards to new employees outside of equity incentive plans.

The following table summarizes information about stock options:

	Number of Options Outstanding		Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Term		Aggregate Intrinsic Value
Balance At September 30, 2016		6,691,200	$	6.56
Granted		638,000		1.95
Cancelled		(1,643,927)		7.02
Exercised		(135,730)		2.01
Balance At September 30, 2017		5,549,543	$	6.00		6.5 years	$	3,390,029
Exercisable At September 30, 2017		4,065,611	$	6.32		5.8 years	$	1,872,018

Stock-based compensation expense related to stock options for the years ended September 30, 2017, 2016 and 2015 was $4,524,833, $6,361,396, and $4,760,831, respectively. The Company does not recognize an income tax benefit as the Company is currently operating at a loss and an actual income tax benefit may not be realized. For non-qualified stock options, the loss creates a timing difference, resulting in a deferred tax asset, which is fully reserved by a valuation allowance.

The grant date fair value of the options granted by the Company for the years ended September 30, 2017, 2016 and 2015 was estimated at $849,816, $6,426,207 and $7,338,395, respectively.

The intrinsic value of the options exercised during the years ended September 30, 2017, 2016 and 2015 was $35,512, $142,690 and $128,391, respectively.

As of September 30, 2017, the pre-tax compensation expense for all outstanding unvested stock options in the amount of approximately $4,627,267 will be recognized in the Company’s results of operations over a weighted average period of 1.8 years.

The fair value of each stock option award is estimated on the date of grant using the Black-Scholes option pricing model. The Black-Scholes option valuation model was developed for use in estimating the fair value of traded options, which do not have vesting restrictions and are fully transferable. The determination of the fair value of each stock option is affected by the Company’s stock price on the date of grant, as well as assumptions regarding a number of highly complex and subjective variables. Because the Company’s employee stock options have characteristics significantly different from those of traded options, and because changes in the subjective input assumptions can materially affect the fair value estimate, in management’s opinion, the existing models do not necessarily provide a reliable single measure of the fair value of its employee stock options.

The assumptions used to value stock options are as follows:

	Years ended September 30,
	2017	2016	2015
Dividend yield	—	—	—
Risk-free interest rate	1.34 – 2.31%	1.05 – 1.89%	1.46 – 1.89%
Volatility	79%	89%	75%
Expected life (in years)	5.85	6.25	6 - 6.25
Weighted average grant date fair value per share of options granted	$1.33	$4.58	$4.24

The dividend yield is zero as the Company currently does not pay a dividend.

The risk-free interest rate is based on that of the U.S. Treasury bond.

Volatility is estimated based on volatility average of the Company’s Common Stock price.

Restricted Stock Units

Restricted stock units (RSUs), including time-based and performance-based awards, were granted under the Company’s 2013 Incentive Plan and as inducement grants granted outside of the Plan. During the year ended September 30, 2017, the Company awarded 2,623,000 restricted stock units to certain members of management. Of the restricted stock units granted during the year ended September 30, 2017, 0 were granted outside of the Plan as an inducement grant to a new employee. At vesting, each RSU will be exchanged for one share of the Company’s Common Stock. Restricted stock unit awards generally vest subject to the satisfaction of service requirements or the satisfaction of both service requirements and achievement of certain performance targets.

The following table summarizes the activity of the Company’s Restricted Stock Units:

	Number of RSUs			Weighted- Average Grant Date Fair Value
Unvested at September 30, 2016		1,356,667		$	6.72
Granted		2,623,000			1.59
Vested		(786,667	)		7.03
Forfeited		(85,000	)		1.55
Unvested at September 30, 2017		3,108,000		$	2.45

During the years ended September 30, 2017, 2016 and 2015, the Company recorded $3,366,762, $5,234,420 and $4,489,931 of expense related to restricted stock units, respectively. Such expense is included in stock-based compensation expense in the Company’s Consolidated Statement of Operations and Comprehensive Loss.

For restricted stock units, the grant date fair value of the award is based on the Company’s closing stock price at the grant date. Expense is recognized over the vesting period for all awards, and commences at the grant date for time-based awards and upon the Company’s determination that the achievement of such performance conditions is probable for performance-based awards.

As of September 30, 2017, the pre-tax compensation expense for all unvested restricted stock units in the amount of approximately $2,448,449 will be recognized in the Company’s results of operations over a weighted average period of 1.1 years.

Fair Value Measurements

12 Months Ended

Sep. 30, 2017

Fair Value Disclosures [Abstract]

Fair Value Measurements

NOTE 9. FAIR VALUE MEASUREMENTS

The Company measures its financial assets and liabilities at fair value. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (i.e., exit price) in an orderly transaction between market participants at the measurement date. Additionally, the Company is required to provide disclosure and categorize assets and liabilities measured at fair value into one of three different levels depending on the assumptions (i.e., inputs) used in the valuation. Level 1 provides the most reliable measure of fair value while Level 3 generally requires significant management judgment. Financial assets and liabilities are classified in their entirety based on the lowest level of input significant to the fair value measurement. The fair value hierarchy is defined as follows:

Level 1—Valuations are based on unadjusted quoted prices in active markets for identical assets or liabilities.

Level 2—Valuations are based on quoted prices for similar assets or liabilities in active markets, or quoted prices in markets that are not active for which significant inputs are observable, either directly or indirectly.

Level 3—Valuations are based on prices or valuation techniques that require inputs that are both unobservable and significant to the overall fair value measurement. Inputs reflect management’s best estimate of what market participants would use in valuing the asset or liability at the measurement date.

The following table summarizes fair value measurements at September 30, 2017 and September 30, 2016 for assets and liabilities measured at fair value on a recurring basis:

September 30, 2017:

	Level 1		Level 2		Level 3		Total
Cash and cash equivalents	$	24,838,567	$	—	$	—	$	24,838,567
Short-term investments	$	40,769,539	$	—	$	—	$	40,769,539
Derivative liabilities	$	—	$	—	$	695,114	$	695,114
Contingent Consideration	$	—	$	—	$	—	$	—

September 30, 2016:

	Level 1		Level 2		Level 3		Total
Cash and cash equivalents	$	85,366,448	$	—	$	—	$	85,366,448
Short-term investments	$	—	$	—	$	—	$	—
Derivative liabilities	$	—	$	—	$	1,602,626	$	1,602,626
Contingent Consideration	$	—	$	—	$	—	$	—

As part of a financing in December 2012, Arrowhead issued warrants to purchase up to 912,543 shares of Common Stock (the “2012 Warrants”) of which 265,161 warrants were outstanding at September 30, 2017. Further, as part of a financing in January 2013, Arrowhead issued warrants to purchase up to 833,530 shares of Common Stock (the “2013 Warrants” and, together with the 2012 Warrants, the “Warrants”) of which 12,123 warrants were outstanding at September 30, 2017. Each of the Warrants contains a mechanism to adjust the strike price upon the issuance of certain dilutive equity securities. If during the terms of the Warrants, the Company issues Common Stock at a price lower than the exercise price for the Warrants, the exercise price would be reduced to the amount equal to the issuance price of the Common Stock. As a result of these features, the Warrants are subject to derivative accounting as prescribed under ASC 815. Accordingly, the fair value of the Warrants on the date of issuance was estimated using an option pricing model and recorded on the Company’s Consolidated Balance Sheet as a derivative liability. The fair value of the Warrants is estimated at the end of each reporting period and the change in the fair value of the Warrants is recorded as a non-operating gain or loss as change in value of derivatives in the Company’s Consolidated Statement of Operations and Comprehensive Loss. During the years ended September 30, 2017, 2016 and 2015 the Company recorded a non-cash gain/(loss) from the change in fair value of the derivative liability of $870,760, $(293,072) and $2,684,712, respectively. Additionally, as part of an equity financing in June 2010, Arrowhead issued warrants to purchase up to 329,649 shares of Common Stock (the “2010 Warrants”), of which warrants to exercise 24,324 shares remained unexercised and were cancelled at their expiration during fiscal 2016.

The assumptions used in valuing the derivative liability were as follows:

2012 Warrants	September 30, 2017	September 30, 2016	September 30, 2015
Risk-free interest rate	1.07%	0.68%	0.6%
Expected life	0.2 Years	1.2 Years	2.2 Years
Dividend yield	—	—	—
Volatility	79%	89%	75%

2013 Warrants	September 30, 2017	September 30, 2016	September 30, 2015
Risk-free interest rate	1.07%	0.68%	0.6%
Expected life	0.3 Years	1.3 Years	2.3 Years
Dividend yield	—	—	—
Volatility	79%	89%	75%

The following is a reconciliation of the derivative liability related to these warrants:

Value at September 30, 2016	$	1,565,874
Issuance of instruments		—
Change in value		(870,760	)
Net settlements		—
Value at September 30, 2017	$	695,114

In conjunction with the financing of Ablaris in fiscal 2011, Arrowhead sold exchange rights to certain investors whereby the investors had the right to exchange their shares of Ablaris for a prescribed number of Arrowhead shares of Common Stock based upon a predefined ratio. The exchange rights had a seven-year term and a current exchange ratio of 0.01. Exchange rights for 675,000 Ablaris shares were sold in fiscal 2011, and 500,000 remained outstanding at September 30, 2016. The 500,000 remaining were exercised during the year ended September 30, 2017. The exchange rights were subject to derivative accounting as prescribed under ASC 815. Accordingly, the fair value of the exchange rights on the date of issuance was estimated using an option pricing model and recorded on the Company’s Consolidated Balance Sheet as a derivative liability. The fair value of the exchange rights was estimated at the end of each reporting period and the change in the fair value of the exchange rights is recorded as a non-operating gain or loss in the Company’s Consolidated Statement of Operations and Comprehensive Loss. During the years ended September 30, 2017, 2016 and 2015, the Company recorded a non-cash gain/(loss) from the change in fair value of the derivative liability of $19,602, $(7,950) and $184,555, respectively. There was no derivative liability remaining at September 30, 2017 given that all of the exchange rights had been exercised.

The derivative assets/liabilities are estimated using option pricing models that are based on the individual characteristics of the warrants or instruments on the valuation date, as well as assumptions for expected volatility, expected life and risk-free interest rate. Changes in the assumptions used could have a material impact on the resulting fair value. The primary input affecting the value of the Company’s derivatives liabilities is the Company’s stock price. Other inputs have a comparatively insignificant effect.

As of September 30, 2015, the Company had a liability for contingent consideration related to its acquisition of the Roche RNAi business completed in 2011. The fair value measurement of the contingent consideration obligations is determined using Level 3 inputs. The fair value of contingent consideration obligations is based on a discounted cash flow model using a probability-weighted income approach. The measurement is based upon unobservable inputs supported by little or no market activity based on the Company’s assumptions and experience. Estimating timing to complete the development and obtain approval of products is difficult, and there are inherent uncertainties in developing a product candidate, such as obtaining U.S. Food and Drug Administration (FDA) and other regulatory approvals. In determining the probability of regulatory approval and commercial success, the Company utilizes data regarding similar milestone events from several sources, including industry studies and its own experience. These fair value measurements represent Level 3 measurements as they are based on significant inputs not observable in the market. Significant judgment is employed in determining the appropriateness of these assumptions as of the acquisition date and for each subsequent period. Accordingly, changes in assumptions could have a material impact on the amount of contingent consideration expense the Company records in any given period. In November 2016, the Company announced the discontinuation of its clinical trial efforts for ARC-520, ARC-AAT and ARC-521. Given this development, the Company assessed the fair value of its contingent consideration obligation to be $0 at September 30, 2016 and 2017.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Income Taxes

12 Months Ended

Sep. 30, 2017

Income Tax Disclosure [Abstract]

Income Taxes

NOTE 10. - INCOME TAXES

The Company utilizes the guidance issued by the FASB for accounting for income taxes which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns.

Under this method, deferred income taxes are recognized for the tax consequences in future years of differences between the tax bases of assets and liabilities and their financial reporting amounts at each year-end based on enacted tax laws and statutory tax rates applicable to the periods in which the differences are expected to affect taxable income. Valuation allowances are established, when necessary, to reduce deferred tax assets to the amount expected to be realized. The provision for income taxes represents the tax payable for the period and the change during the period in deferred tax assets and liabilities.

Components of the net deferred tax asset (liability) at September 30, 2017 and 2016 are as follows:

		2017		2016
Deferred tax assets:
Accrued compensation	$	1,337,619		$	1,691,050
Stock compensation		5,670,468			7,224,958
Capitalized research and development		2,196,542			2,957,528
Fixed Assets		203,477			—
Net operating losses		132,119,837			125,120,957
Intangible Assets		5,358,700			5,544,091
Deferred Revenue		1,068,206			—
Deferred Rent		865,675			—
Capital Loss		1,020,162			—
Total deferred tax assets		149,840,686			142,538,584
Valuation allowance		(136,625,436	)		(128,695,371	)
Deferred tax liabilities:
State taxes		(13,215,250	)		(13,804,268	)
Fixed assets		—			(38,945	)
Total deferred tax liability		(13,215,250	)		(13,843,213	)
Net deferred tax assets (liabilities)	$	—		$	—

The Company has concluded, in accordance with the applicable accounting standards, that it is more likely than not that the Company may not realize the benefit of all of its deferred tax assets. Accordingly, management has provided a 100% valuation allowance against its deferred tax assets until such time as management believes that its projections of future profits as well as expected future tax rates make the realization of these deferred tax assets more-likely-than-not. Significant judgment is required in the evaluation of deferred tax benefits and differences in future results from our estimates could result in material differences in the realization of these assets. The Company has recorded a full valuation allowance related to all of its deferred tax assets. The Company has performed an assessment of positive and negative evidence regarding the realization of the net deferred tax asset in accordance with FASB ASC 740-10, “Accounting for Income Taxes.” This assessment included the evaluation of scheduled reversals of deferred tax liabilities, the availability of carry forwards and estimates of projected future taxable income.

As of September 30, 2016, the Company had available gross federal net operating loss (NOL) carry forwards of $260.7 million and gross state NOL carry forwards of $398.3 million. Gross federal NOL carry forwards for 2017 are estimated at $24.5 million, and gross state NOL carry forwards for 2017 are estimated at $24.2 million. The NOLs expire at various dates through 2037.

The provisions for income taxes for the years ended September 30, 2017 and 2016 are as follows:

	2017		2016
Federal:
Current		—		—
Deferred		—		—
Total Federal		—		—
State:
Current	$	2,400		2,400
Deferred		—		—
Total State	$	2,400		2,400
Provision from income taxes	$	2,400		2,400

The Company’s effective income tax rate differs from the statutory federal income tax rate as follows for the years ended September 30, 2017 and 2016:

	2017		2016
At U.S. federal statutory rate	34.0	%		34.0	%
State taxes, net of federal effect	4.9			9.0
Stock compensation	(5.8	)		(1.1	)
Mark-to-market adjustments	0.9			(0.1	)
Valuation allowance	(23.1	)		(41.1	)
True-up on deferred taxes	(3.0	)		—
State blended rate change	(5.7	)		—
Other	(2.2	)		(0.7	)
Effective income tax rate	0.0	%		0.0	%

The Company has adopted guidance issued by the FASB that clarifies the accounting for uncertainty in income taxes recognized in an enterprise’s financial statements and prescribes a recognition threshold of more likely than not and a measurement process for financial statement recognition and measurement of a tax position taken or expected to be taken in a tax return. In making this assessment, a company must determine whether it is more likely than not that a tax position will be sustained upon examination, based solely on the technical merits of the position and must assume that the tax position will be examined by taxing authorities. The Company’s policy is to include interest and penalties related to unrecognized tax benefits in income tax expense. The Company has not recognized any unrecognized tax benefits and does not have any interest or penalties related to uncertain tax positions as of September 30, 2017 and 2016.

The Company files income tax returns with the Internal Revenue Service (“IRS”), the state of California, the Australia Tax Office (“ATO”) and certain other taxing jurisdictions. The Company is subject to income tax examinations by the IRS and by state tax authorities until the net operating losses are settled. During the three months ended September 30, 2016, the IRS commenced an audit for the tax year ended September 30, 2015. The audit concluded during the year ended September 30, 2017 with no material adjustments. In November 2017, the ATO commenced an audit for the tax year ended September 30, 2016.

Employee Benefit Plans	12 Months Ended
Employee Benefit Plans	Sep. 30, 2017
Compensation And Retirement Disclosure [Abstract]
Employee Benefit Plans	NOTE 11. EMPLOYEE BENEFIT PLANS In January 2005, the Company adopted a defined contribution 401(k) retirement savings plan covering substantially all of its employees. The Plan is administered under the “safe harbor” provision of ERISA. Under the terms of the plan, an eligible employee may elect to contribute a portion of their salary on a pre-tax basis, subject to federal statutory limitations. The plan allows for a discretionary match in an amount up to 100% of each participant’s first 3% of compensation contributed plus 50% of each participant’s next 2% of compensation contributed. For the years ended September 30, 2017, 2016, and 2015, we recorded expenses under this plan of approximately $426,470, $476,835 and $407,603, respectively. In addition to the employee benefit plans described above, the Company provides certain employee benefit plans, including those which provide health and life insurance benefits to employees.

Unaudited Quarterly Financial Data

12 Months Ended

Sep. 30, 2017

Quarterly Financial Information Disclosure [Abstract]

Unaudited Quarterly Financial Data

NOTE 12. UNAUDITED QUARTERLY FINANCIAL DATA

The following table presents selected unaudited quarterly financial data for each full quarterly period of the years ended September 30, 2017 and 2016:

	First		Second		Third		Fourth
Year ended September 30, 2017	Quarter		Quarter		Quarter		Quarter
Revenues	$	4,365,496	$	8,985,930	$	9,342,498	$	8,713,785
Operating Losses	$	(14,901,887)	$	(6,129,642)	$	(5,714,164)	$	(10,250,216)
Net Loss	$	(12,086,108)	$	(6,042,557)	$	(5,519,741)	$	(10,731,889)
Net Loss Attributable to Arrowhead	$	(12,086,108)	$	(6,042,557)	$	(5,519,741)	$	(10,731,889)
Loss per share (Basic and Diluted)	$	(0.17)	$	(0.08)	$	(0.07)	$	(0.14)

	First		Second		Third		Fourth
Year ended September 30, 2016	Quarter		Quarter		Quarter		Quarter
Revenues	$	43,750	$	43,750	$	39,583	$	31,250
Operating Losses	$	(19,341,270)	$	(21,264,855)	$	(19,341,487)	$	(21,795,114)
Net Loss	$	(19,264,414)	$	(20,815,860)	$	(19,420,743)	$	(22,221,985)
Net Loss Attributable to Arrowhead	$	(19,264,414)	$	(20,815,860)	$	(19,420,743)	$	(22,221,985)
Loss per share (Basic and Diluted)	$	(0.32)	$	(0.35)	$	(0.32)	$	(0.34)

Organization and Significant Accounting Policies (Policies)	12 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation—The consolidated financial statements include the accounts of Arrowhead and its Subsidiaries. Arrowhead’s primary operating subsidiary is Arrowhead Madison, which is located in Madison, Wisconsin, where the Company’s research and development facility is located. All significant intercompany accounts and transactions are eliminated in consolidation.
Basis of Presentation and Use of Estimates	Basis of Presentation and Use of Estimates—The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could materially differ from those estimates. Additionally, certain reclassifications have been made to prior period financial statements to conform to the current period presentation.
Cash and Cash Equivalents	Cash and Cash Equivalents—The Company considers all liquid debt instruments purchased with a maturity of three months or less to be cash equivalents. The Company had no restricted cash at September 30, 2017 and September 30, 2016.
Concentration of Credit Risk	Concentration of Credit Risk—The Company maintains several bank accounts at two financial institutions for its operations. These accounts are insured by the Federal Deposit Insurance Corporation (FDIC) for up to $250,000 per institution. Management believes the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which these deposits are held.
Investments	Investments—The Company may invest excess cash balances in short-term and long-term marketable debt securities. Investments may include certificates of deposits, money market accounts, government-sponsored enterprise securities, corporate bonds and/or commercial paper. The Company accounts for its investment in marketable securities in accordance with FASB ASC 320, Investments – Debt and Equity Securities. This statement requires certain securities to be classified into three categories: Held-to-maturity—Debt securities that the entity has the positive intent and ability to hold to maturity are reported at amortized cost. Trading Securities—Debt and equity securities that are bought and held primarily for the purpose of selling in the near term are reported at fair value, with unrealized gains and losses included in earnings. Available-for-Sale—Debt and equity securities not classified as either securities held-to-maturity or trading securities are reported at fair value with unrealized gains or losses excluded from earnings and reported as a separate component of shareholders’ equity. The Company classifies its investments in marketable debt securities based on the facts and circumstances present at the time of purchase of the securities. During the year ended September 30, 2017, all of the Company’s investments were classified as held-to-maturity. Held-to-maturity investments are measured and recorded at amortized cost on the Company’s Consolidated Balance Sheet. Discounts and premiums to par value of the debt securities are amortized to interest income/expense over the term of the security. No gains or losses on investment securities are realized until they are sold or a decline in fair value is determined to be other-than-temporary.
Property and Equipment	Property and Equipment—Property and equipment are recorded at cost, which may equal fair market value in the case of property and equipment acquired in conjunction with a business acquisition. Depreciation of property and equipment is recorded using the straight-line method over the respective useful lives of the assets ranging from three to seven years. Leasehold improvements are amortized over the lesser of the expected useful life or the remaining lease term. Long-lived assets, including property and equipment are reviewed for impairment whenever events or circumstances indicate that the carrying amount of these assets may not be recoverable.
Intangible Assets subject to amortization	Intangible Assets Subject to Amortization—Intangible assets subject to amortization include certain patents and a license agreement. Intangible assets subject to amortization are reviewed for impairment whenever events or circumstances indicate that the carrying amount of these assets may not be recoverable.
Contingent Consideration	Contingent Consideration - The consideration for the Company’s acquisitions may include potential future payments that are contingent upon the occurrence of a particular event. For example, milestone payments might be based on the achievement of various regulatory approvals or future sales milestones, and royalty payments might be based on drug product sales levels. The Company records a contingent consideration obligation for such contingent payments at fair value on the acquisition date. The Company estimates the fair value of contingent consideration obligations through valuation models designed to estimate the probability of such contingent payments based on various assumptions and incorporating estimated success rates. Estimated payments are discounted using present value techniques to arrive at an estimated fair value at the balance sheet date. Changes in the fair value of the contingent consideration obligations are recognized within the Company’s Consolidated Statements of Operations and Comprehensive Loss. Changes in the fair value of the contingent consideration obligations can result from changes to one or multiple inputs, including adjustments to the discount rates, changes in the amount or timing of expected expenditures associated with product development, changes in the amount or timing of cash flows from products upon commercialization, changes in the assumed achievement or timing of any development milestones, changes in the probability of certain clinical events and changes in the assumed probability associated with regulatory approval. These fair value measurements are based on significant inputs not observable in the market. Substantial judgment is employed in determining the appropriateness of these assumptions as of the acquisition date and for each subsequent period. Accordingly, changes in assumptions could have a material impact on the amount of contingent consideration expense the Company records in any given period. The Company determined the fair value of its contingent consideration obligation to be $0 at September 30, 2017 and September 30, 2016, given the discontinuation of its clinical trials.
Revenue Recognition	Revenue Recognition— Revenue from product sales is recorded when persuasive evidence of an arrangement exists, title has passed and delivery has occurred, a price is fixed and determinable, and collection is reasonably assured. The Company may generate revenue from technology licenses, collaborative research and development arrangements, and product sales. Revenue under technology licenses and collaborative agreements typically consists of nonrefundable and/or guaranteed technology license fees, collaborative research funding, manufacturing and development services and various milestone and future product royalty or profit-sharing payments. These agreements are considered to be multiple element arrangements. The Company applies the accounting standard on revenue recognition for multiple element arrangements. The fair value of deliverables under the arrangement may be derived using a best estimate of selling price if vendor specific objective evidence and third-party evidence is not available. Deliverables under the arrangement will be separate units of accounting if a delivered item has value to the customer on a standalone basis, if the arrangement includes a general right of return for the delivered item, and if delivery or performance of the undelivered item is considered probable and substantially in the Company’s control. The Company recognizes upfront license payments as revenue upon delivery of the license only if the license has standalone value from any undelivered performance obligations and that value can be determined. The undelivered performance obligations typically include manufacturing or development services or research and/or steering committee services. If the fair value of the undelivered performance obligations can be determined, then these obligations would be accounted for separately. If the license is not considered to have standalone value, then the license and other undelivered performance obligations would be accounted for as a single unit of accounting. In this case, the license payments and payments for performance obligations are recognized as revenue over the estimated period of when the performance obligations are performed or deferred indefinitely until the undelivered performance obligation is determined. Whenever the Company determines that an arrangement should be accounted for as a single unit of accounting, the Company determines the period over which the performance obligations will be performed and revenue will be recognized. Revenue is recognized using a proportional performance or straight-line method. The proportional performance method is used when the level of effort required to complete performance obligations under an arrangement can be reasonably estimated. The amount of revenue recognized under the proportional performance method is determined by multiplying the total payments under the contract, excluding royalties and payments contingent upon achievement of milestones, by the ratio of the level of effort performed to date to the estimated total level of effort required to complete performance obligations under the arrangement. If the Company cannot reasonably estimate the level of effort to complete performance obligations under an arrangement, the Company recognizes revenue under the arrangement on a straight-line basis over the period the Company is expected to complete its performance obligations. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company is expected to complete its performance obligations under an arrangement. Many of the Company’s collaboration agreements entitle the Company to additional payments upon the achievement of development, regulatory and sales performance-based milestones. If the achievement of a milestone is considered probable at the inception of the collaboration agreement, the related milestone payment is included with other collaboration consideration, such as upfront fees and research funding, in the Company’s revenue calculation. Typically these milestones are not considered probable at the inception of the collaboration. As such, milestones will typically be recognized in one of two ways depending on the timing of when the milestone is achieved. If the milestone is achieved during the performance period, the Company will only recognize revenue to the extent of the proportional performance achieved at that date, or the proportion of the straight-line basis achieved at that date, and the remainder will be recorded as deferred revenue to be amortized over the remaining performance period. If the milestone is achieved after the performance period has completed and all performance obligations have been delivered, the Company will recognize the milestone payment as revenue in its entirety in the period the milestone was achieved. Deferred revenue will be classified as part of Current or Long-Term Liabilities in the accompanying Consolidated Balance Sheets based on the Company’s estimate of the portion of the performance obligations regarding that revenue will be completed within the next 12 months divided by the total performance period estimate. This estimate is based on the Company’s current operating plan and, if the Company’s operating plan should change in the future, the Company may recognize a different amount of deferred revenue over the next 12-month period.
Allowance for Doubtful Accounts	Allowance for Doubtful Accounts—The Company accrues an allowance for doubtful accounts based on estimates of uncollectible revenues by analyzing historical collections, accounts receivable aging and other factors. Accounts receivable are written off when all collection attempts have failed.
Research and Development	Research and Development—Costs and expenses that can be clearly identified as research and development are charged to expense as incurred in accordance with FASB ASC 730-10. Included in research and development costs are operating costs, facilities, supplies, external services, clinical trial and manufacturing costs, overhead directly related to the Company’s research and development operations, and costs to acquire technology licenses.
Earnings (Loss) per Share	Earnings (Loss) per Share—Basic earnings (loss) per share is computed using the weighted-average number of common shares outstanding during the period. Diluted earnings (loss) per share are computed using the weighted-average number of common shares and dilutive potential common shares outstanding during the period. Dilutive potential common shares primarily consist of stock options and restricted stock units issued to employees and warrants to purchase Common Stock of the Company. All outstanding stock options, restricted stock units and warrants for the years ended September 30, 2017, 2016 and 2015 have been excluded from the calculation of Diluted earnings (loss) per share due to their anti-dilutive effect.
Stock-Based Compensation	Stock-Based Compensation—The Company accounts for share-based compensation arrangements in accordance with FASB ASC 718, which requires the measurement and recognition of compensation expense for all share-based payment awards to be based on estimated fair values. The Company uses the Black-Scholes option valuation model to estimate the fair value of its stock options at the date of grant. The Black-Scholes option valuation model requires the input of subjective assumptions to calculate the value of stock options. For restricted stock units, the value of the award is based on the Company’s stock price at the grant date. For performance-based restricted stock unit awards, the value of the award is based on the Company’s stock price at the grant date. The Company uses historical data and other information to estimate the expected price volatility for stock option awards and the expected forfeiture rate for all awards. Expense is recognized over the vesting period for all awards, and commences at the grant date for time-based awards and upon the Company’s determination that the achievement of such performance conditions is probable for performance-based awards. This determination requires significant judgment by management.
Derivative Assets and Liabilities	Derivative Assets and Liabilities – The Company accounts for warrants and other derivative financial instruments as either equity or assets/liabilities based upon the characteristics and provisions of each instrument. Warrants classified as equity are recorded as Additional Paid-In Capital on the Company’s Consolidated Balance Sheet. Some of the Company’s warrants were determined to be ineligible for equity classification due to provisions that may result in an adjustment to their exercise price. Warrants classified as derivative liabilities and other derivative financial instruments that require separate accounting as assets or liabilities are recorded on the Company’s Consolidated Balance Sheet at their fair value on the date of issuance and are revalued on each subsequent balance sheet date until such instruments are exercised or expire, with any changes in the fair value between reporting periods recorded as Other Income or Expense. The Company estimates the fair value of these assets/liabilities using option pricing models that are based on the individual characteristics of the warrants or instruments on the valuation date, as well as assumptions for expected volatility, expected life and risk-free interest rate.
Income Taxes	Income Taxes—The Company accounts for income taxes under the liability method, which requires the recognition of deferred income tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Under this method, deferred income taxes are recognized for the tax consequences in future years of differences between the tax bases of assets and liabilities and their financial reporting amounts at each period end based on enacted tax laws and statutory tax rates applicable to the periods in which the differences are expected to affect taxable income. Valuation allowances are established, when necessary, to reduce deferred income tax assets to the amount expected to be realized. The provision for income taxes, if any, represents the tax payable for the period and the change in deferred income tax assets and liabilities during the period.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09 Revenue from Contracts with Customers (Topic 606), which will supersede nearly all existing revenue recognition guidance under GAAP. ASU No. 2014-09 provides that an entity recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. This update also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments, and assets recognized from costs incurred to obtain or fulfill a contract. ASU No. 2014-09 allows for either full retrospective or modified retrospective adoption and will become effective for the Company in the first quarter of 2019. The Company is evaluating the potential effects of the adoption of this update on its financial statements. In March 2016, the FASB issued ASU No. 2016-02, Leases. Under ASU 2016-02, lessees will be required to recognize a right-of-use asset and a lease liability for virtually all of their leases (other than leases that meet the definition of a short-term lease). For income statement purposes, a dual model was retained, requiring leases to be classified as either operating or finance. Operating leases will result in straight-line expense (similar to current operating leases) while finance leases will result in a front-loaded expense pattern (similar to current capital leases). ASU 2016-02 becomes effective for the Company in the first quarter of fiscal 2020. The Company expects the adoption of this update to have a material effect on the classification and disclosure of its leased facilities in Madison, Wisconsin. In May 2017, the FASB issued ASU No. 2017-09, which is an update to Topic 718, Compensation - Stock Compensation. The update provides guidance on determining which changes to the terms and conditions of share-based payment awards, including stock options, require an entity to apply modification accounting under Topic 718. ASU 2017-09 becomes effective for the Company in the first quarter of fiscal 2019. The Company does not expect that ASU 2017-09 will have a material impact on the Company's results of operations and consolidated financial statements.

Property and Equipment (Tables)

12 Months Ended

Sep. 30, 2017

Property Plant And Equipment [Abstract]

Summary of Property and Equipment

The following table summarizes the Company’s major classes of property and equipment:


	September 30, 2017			September 30, 2016
Computers, office equipment and furniture	$	600,334		$	442,915
Research equipment		9,660,960			7,490,400
Software		132,078			80,841
Leasehold improvements		12,208,380			11,885,365
Total gross fixed assets		22,601,752			19,899,521
Less: Accumulated depreciation and amortization		(7,088,733	)		(4,512,760	)
Property and equipment, net	$	15,513,019		$	15,386,761

Investments (Tables)

12 Months Ended

Sep. 30, 2017

Equity [Abstract]

Summary of Short-term Investments

The following tables summarize the Company’s short-term investments as of September 30, 2017, and September 30, 2016.

	As of September 30, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Commercial notes (due within one year)	$	40,769,539	$	—	$	(334,755)	$	40,434,784

As of September 30, 2016

Amortized Cost

Gross Unrealized Gains

Gross Unrealized Losses

Fair Value

Commercial notes (due within one year)

—

- Definition

Tabular disclosure of all investments in certain debt and equity securities for which the entity has the positive intent and ability to hold until maturity.

+ References + Details

Intangible Assets (Tables)

12 Months Ended

Sep. 30, 2017

Goodwill And Intangible Assets Disclosure [Abstract]

Schedule of Intangible Assets

The following table provides details on the Company’s intangible asset balances:

	Intangible assets subject to amortization
Balance at September 30, 2016		$	22,164,868
Impairment			-
Amortization			(1,700,429	)
Balance at September 30, 2017		$	20,464,439

Stockholders' Equity (Tables)

12 Months Ended

Sep. 30, 2017

Equity [Abstract]

Summary of Information About Warrants

The following table summarizes information about warrants outstanding at September 30, 2017:

Exercise prices		Number of Warrants		Remaining Life in Years
$	2.12		75,000		0.4
$	1.83		277,284		0.2
$	7.14		80,000		0.7
Total warrants outstanding			432,284

- Definition

Tabular disclosure of warrants or rights issued. Warrants and rights outstanding are derivative securities that give the holder the right to purchase securities (usually equity) from the issuer at a specific price within a certain time frame. Warrants are often included in a new debt issue to entice investors by a higher return potential. The main difference between warrants and call options is that warrants are issued and guaranteed by the company, whereas options are exchange instruments and are not issued by the company. Also, the lifetime of a warrant is often measured in years, while the lifetime of a typical option is measured in months. Disclose the title of issue of securities called for by warrants and rights outstanding, the aggregate amount of securities called for by warrants and rights outstanding, the date from which the warrants or rights are exercisable, and the price at which the warrant or right is exercisable.

+ References + Details

Commitments and Contingencies (Tables)

12 Months Ended

Sep. 30, 2017

Commitments And Contingencies Disclosure [Abstract]

Future Minimum Lease Payments Under Operating Leases

As of September 30, 2017, future minimum lease payments due in fiscal years under operating leases are as follows:

2018	$	1,531,234
2019		1,435,409
2020		1,044,431
2021		1,070,496
2022		1,097,168
2023 and thereafter		4,669,328
Total	$	10,848,066

Future Principal Payments Under Note Payable

As of September 30, 2017, future principal payments due in fiscal years under the note payable are as follows:

2018	$	208,506
2019		223,820
2020		240,258
2021		257,903
2022		276,845
2023 and thereafter		1,326,192
Total	$	2,533,524

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e),(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21538-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21521-112644

+ Details

Stock-Based Compensation (Tables)

12 Months Ended

Sep. 30, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summarized Information about Stock Options

The following table summarizes information about stock options:

	Number of Options Outstanding		Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Term		Aggregate Intrinsic Value
Balance At September 30, 2016		6,691,200	$	6.56
Granted		638,000		1.95
Cancelled		(1,643,927)		7.02
Exercised		(135,730)		2.01
Balance At September 30, 2017		5,549,543	$	6.00		6.5 years	$	3,390,029
Exercisable At September 30, 2017		4,065,611	$	6.32		5.8 years	$	1,872,018

Assumptions Used to Value Stock Options

The assumptions used to value stock options are as follows:

	Years ended September 30,
	2017	2016	2015
Dividend yield	—	—	—
Risk-free interest rate	1.34 – 2.31%	1.05 – 1.89%	1.46 – 1.89%
Volatility	79%	89%	75%
Expected life (in years)	5.85	6.25	6 - 6.25
Weighted average grant date fair value per share of options granted	$1.33	$4.58	$4.24

Summary of Share Activity Related to Restricted Stock Units

The following table summarizes the activity of the Company’s Restricted Stock Units:

	Number of RSUs			Weighted- Average Grant Date Fair Value
Unvested at September 30, 2016		1,356,667		$	6.72
Granted		2,623,000			1.59
Vested		(786,667	)		7.03
Forfeited		(85,000	)		1.55
Unvested at September 30, 2017		3,108,000		$	2.45

Fair Value Measurements (Tables)

12 Months Ended

Sep. 30, 2017

Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]

Fair Value Measurements for Assets and Liabilities Measured at Fair Value on Recurring Basis

The following table summarizes fair value measurements at September 30, 2017 and September 30, 2016 for assets and liabilities measured at fair value on a recurring basis:

September 30, 2017:

	Level 1		Level 2		Level 3		Total
Cash and cash equivalents	$	24,838,567	$	—	$	—	$	24,838,567
Short-term investments	$	40,769,539	$	—	$	—	$	40,769,539
Derivative liabilities	$	—	$	—	$	695,114	$	695,114
Contingent Consideration	$	—	$	—	$	—	$	—

September 30, 2016:

	Level 1		Level 2		Level 3		Total
Cash and cash equivalents	$	85,366,448	$	—	$	—	$	85,366,448
Short-term investments	$	—	$	—	$	—	$	—
Derivative liabilities	$	—	$	—	$	1,602,626	$	1,602,626
Contingent Consideration	$	—	$	—	$	—	$	—

Warrant

Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]

Assumptions Used in Valuing Derivative Liabilities

The assumptions used in valuing the derivative liability were as follows:

2012 Warrants	September 30, 2017	September 30, 2016	September 30, 2015
Risk-free interest rate	1.07%	0.68%	0.6%
Expected life	0.2 Years	1.2 Years	2.2 Years
Dividend yield	—	—	—
Volatility	79%	89%	75%

2013 Warrants	September 30, 2017	September 30, 2016	September 30, 2015
Risk-free interest rate	1.07%	0.68%	0.6%
Expected life	0.3 Years	1.3 Years	2.3 Years
Dividend yield	—	—	—
Volatility	79%	89%	75%

Reconciliation of Derivative Liability

The following is a reconciliation of the derivative liability related to these warrants:

Value at September 30, 2016	$	1,565,874
Issuance of instruments		—
Change in value		(870,760	)
Net settlements		—
Value at September 30, 2017	$	695,114

Income Taxes (Tables)

12 Months Ended

Sep. 30, 2017

Income Tax Disclosure [Abstract]

Components of the Net Deferred Tax (Liability)

Components of the net deferred tax asset (liability) at September 30, 2017 and 2016 are as follows:

		2017		2016
Deferred tax assets:
Accrued compensation	$	1,337,619		$	1,691,050
Stock compensation		5,670,468			7,224,958
Capitalized research and development		2,196,542			2,957,528
Fixed Assets		203,477			—
Net operating losses		132,119,837			125,120,957
Intangible Assets		5,358,700			5,544,091
Deferred Revenue		1,068,206			—
Deferred Rent		865,675			—
Capital Loss		1,020,162			—
Total deferred tax assets		149,840,686			142,538,584
Valuation allowance		(136,625,436	)		(128,695,371	)
Deferred tax liabilities:
State taxes		(13,215,250	)		(13,804,268	)
Fixed assets		—			(38,945	)
Total deferred tax liability		(13,215,250	)		(13,843,213	)
Net deferred tax assets (liabilities)	$	—		$	—

Provisions for Income Taxes

The provisions for income taxes for the years ended September 30, 2017 and 2016 are as follows:

	2017		2016
Federal:
Current		—		—
Deferred		—		—
Total Federal		—		—
State:
Current	$	2,400		2,400
Deferred		—		—
Total State	$	2,400		2,400
Provision from income taxes	$	2,400		2,400

Summary of Effective Income Tax Rate Reconciliation

The Company’s effective income tax rate differs from the statutory federal income tax rate as follows for the years ended September 30, 2017 and 2016:

	2017		2016
At U.S. federal statutory rate	34.0	%		34.0	%
State taxes, net of federal effect	4.9			9.0
Stock compensation	(5.8	)		(1.1	)
Mark-to-market adjustments	0.9			(0.1	)
Valuation allowance	(23.1	)		(41.1	)
True-up on deferred taxes	(3.0	)		—
State blended rate change	(5.7	)		—
Other	(2.2	)		(0.7	)
Effective income tax rate	0.0	%		0.0	%

Unaudited Quarterly Financial Data (Tables)

12 Months Ended

Sep. 30, 2017

Quarterly Financial Information Disclosure [Abstract]

Schedule of Unaudited Quarterly Financial Data

The following table presents selected unaudited quarterly financial data for each full quarterly period of the years ended September 30, 2017 and 2016:

	First		Second		Third		Fourth
Year ended September 30, 2017	Quarter		Quarter		Quarter		Quarter
Revenues	$	4,365,496	$	8,985,930	$	9,342,498	$	8,713,785
Operating Losses	$	(14,901,887)	$	(6,129,642)	$	(5,714,164)	$	(10,250,216)
Net Loss	$	(12,086,108)	$	(6,042,557)	$	(5,519,741)	$	(10,731,889)
Net Loss Attributable to Arrowhead	$	(12,086,108)	$	(6,042,557)	$	(5,519,741)	$	(10,731,889)
Loss per share (Basic and Diluted)	$	(0.17)	$	(0.08)	$	(0.07)	$	(0.14)

	First		Second		Third		Fourth
Year ended September 30, 2016	Quarter		Quarter		Quarter		Quarter
Revenues	$	43,750	$	43,750	$	39,583	$	31,250
Operating Losses	$	(19,341,270)	$	(21,264,855)	$	(19,341,487)	$	(21,795,114)
Net Loss	$	(19,264,414)	$	(20,815,860)	$	(19,420,743)	$	(22,221,985)
Net Loss Attributable to Arrowhead	$	(19,264,414)	$	(20,815,860)	$	(19,420,743)	$	(22,221,985)
Loss per share (Basic and Diluted)	$	(0.32)	$	(0.35)	$	(0.32)	$	(0.34)

Organization and Significant Accounting Policies - Additional Information (Detail) - USD ($)		12 Months Ended
	Nov. 18, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Organization And Significant Accounting Policies [Line Items]
Cash and cash equivalents		$ 24,838,567	$ 85,366,448	$ 81,214,354	$ 132,510,610
Short term investments		40,769,539
Net increase (decrease) in cash and investments		(19,800,000)
Cash outflows related to operating activities excluding upfront payment from Amgen		(53,900,000)
Capital expenditures		7,918,198	3,860,237	$ 1,970,612
Proceeds from the issuance of common stock		12,419,119	52,231,433
Restricted Cash		0	0
Fair value of contingent consideration obligation due to discontinuation of clinical trials		0	$ 0
Maximum
Organization And Significant Accounting Policies [Line Items]
Amount insured in FDIC per account		$ 250,000
Property, Plant and Equipment, Useful Life		7 years
Minimum
Organization And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life		3 years
Amgen
Organization And Significant Accounting Policies [Line Items]
Proceeds from upfront payments and equity investments		$ 42,500,000
Amgen \| Common Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock	$ 12,500,000
Amgen \| ARO-LPA (AMG 890) Agreement
Organization And Significant Accounting Policies [Line Items]
Upfront payment	$ 30,000,000
Amgen \| ARO-LPA (AMG 890) Agreement \| Common Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Common stock issued	1,745,810

Collaboration and License Agreements - Amgen, Inc - Additional Information (Detail)				12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares	Nov. 18, 2016 USD ($)	Sep. 28, 2016 USD ($) Agreement	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock				$ 12,419,119	$ 52,231,433
Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Stock issued, price per share \| $ / shares	$ 7.16			$ 7.16	$ 7.16
Collaboration and License agreements \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Number of agreements \| Agreement			2
ARO-AMG1 Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional amount of common stock shares agreed to sell upon exercise of option			$ 5,000,000
Number of trading days, used to calculate share price of Common Stock, surrounding option exercise date			30 days
Deferred revenue	$ 5,000,000			$ 5,000,000
Deferred revenue amortized into revenue				2,500,000
Collaboration and License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash received as due under collaboration agreement			$ 35,000,000
Collaboration and License agreements \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option payments, and development, regulatory and sales milestone payments.			617,000,000
Collaboration and License agreements \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock			$ 21,500,000
ARO-LPA (AMG 890) Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 30,000,000			30,000,000
Deferred revenue amortized into revenue				27,300,000
Revenue generated from work orders under services agreement				$ 1,500,000
Common Stock Purchase Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Shares issued \| shares	3,002,793			3,002,793
Stock issued, price per share \| $ / shares	$ 7.16			$ 7.16
Proceeds from the issuance of common stock		$ 12,500,000
Common Stock Purchase Agreement \| Amgen \| NASDAQ
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of trading days, used to calculate weighted average price of Common Stock, listed in stock market	30 days

Property and Equipment - Summary of Property and Equipment (Detail) - USD ($)	Sep. 30, 2017	Sep. 30, 2016
Property Plant And Equipment [Abstract]
Computers, office equipment and furniture	$ 600,334	$ 442,915
Research equipment	9,660,960	7,490,400
Software	132,078	80,841
Leasehold improvements	12,208,380	11,885,365
Total gross fixed assets	22,601,752	19,899,521
Less: Accumulated depreciation and amortization	(7,088,733)	(4,512,760)
Property and equipment, net	$ 15,513,019	$ 15,386,761

Investments - Summary of Short-term Investments (Detail)	Sep. 30, 2017 USD ($)
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	$ 40,769,539
Commercial Notes Due Within One Year
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	40,769,539
Gross Unrealized Losses	(334,755)
Fair Value	$ 40,434,784

Intangible Assets - Additional Information (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Finite Lived Intangible Assets [Line Items]
Amortization expense	$ 1,700,429	$ 1,714,313	$ 1,046,571
Amortization of license agreements in 2018	1,700,429
Amortization of license agreements in 2019	1,700,429
Amortization of license agreements in 2020	1,700,429
Amortization of license agreements in 2021	1,700,429
Amortization of license agreements in 2022	1,700,429
Amortization of license agreements, thereafter	$ 11,962,294
Novartis \| Licensing Agreement
Finite Lived Intangible Assets [Line Items]
Amortization period of intangible assets	21 years
Finite-lived intangible assets, accumulated amortization	$ 383,330
Novartis \| Patents
Finite Lived Intangible Assets [Line Items]
Amortization period of intangible assets	14 years
Finite-lived intangible assets, accumulated amortization	$ 4,009,395

Intangible Assets - Schedule of Intangible Assets (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Intangible Assets Net Excluding Goodwill [Abstract]
Intangible assets subject to amortization, beginning balance	$ 22,164,868
Intangible assets subject to amortization, Amortization	(1,700,429)	$ (1,714,313)	$ (1,046,571)
Intangible assets subject to amortization, ending balance	$ 20,464,439	$ 22,164,868

Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended	12 Months Ended
	Nov. 18, 2016	Sep. 28, 2016	Nov. 30, 2016	Sep. 30, 2017	Mar. 21, 2017	Sep. 30, 2016
Class Of Stock [Line Items]
Capital stock authorized for issuance				150,000,000
Common stock, shares authorized				145,000,000		145,000,000
Common stock, par value				$ 0.001		$ 0.001
Preferred stock, shares authorized				5,000,000		5,000,000
Preferred stock, par value				$ 0.001		$ 0.001
Common stock, shares outstanding				74,785,426		69,746,685
Preferred stock, shares outstanding				0		15,652
Dividend distribution, declared date				Mar. 21, 2017
Number of rights distributed for each outstanding share of common stock					1
Dividend distribution, record date				Mar. 22, 2017
Rights expiration date				Mar. 21, 2018
Common Stock Purchase Agreement \| Amgen \| First Tranche
Class Of Stock [Line Items]
Common stock shares sold		1,256,983
Common stock shares sold, price per share		$ 7.16
Net proceeds received		$ 9.0
Common Stock Purchase Agreement \| Amgen \| Second Tranche
Class Of Stock [Line Items]
Common stock shares sold	1,745,810
Common stock shares sold, price per share	$ 7.16
Net proceeds received			$ 12.5
Series C Preferred Stock
Class Of Stock [Line Items]
Preferred stock, shares converted				15,652
Common stock, issued upon conversion of preferred stock				2,670,989
Preferred stock, shares outstanding				0
Series D Junior Participating Preferred
Class Of Stock [Line Items]
Preferred stock, par value					$ 0.001
Preferred stock conversion of purchase price per one one thousandth of share					$ 20
Beneficial ownership percentage of common stock upon right exercisable					15.00%
Number of rights distributed for each outstanding share of common stock				1
2004 Equity Incentive Plan, 2013 Equity Incentive Plan, and Inducement Grants
Class Of Stock [Line Items]
Common Stock, Share reserve for issuance				8,902,416

Stockholders' Equity - Summary of Information About Warrants (Detail)	12 Months Ended
	Sep. 30, 2017 $ / shares shares
Class Of Warrant Or Right [Line Items]
Warrants outstanding	432,284
Warrant 1
Class Of Warrant Or Right [Line Items]
Exercise prices \| $ / shares	$ 2.12
Warrants outstanding	75,000
Remaining Life in Years	4 months 24 days
Warrant 2
Class Of Warrant Or Right [Line Items]
Exercise prices \| $ / shares	$ 1.83
Warrants outstanding	277,284
Remaining Life in Years	2 months 12 days
Warrant 3
Class Of Warrant Or Right [Line Items]
Exercise prices \| $ / shares	$ 7.14
Warrants outstanding	80,000
Remaining Life in Years	8 months 12 days

Commitments and Contingencies - Additional Information (Detail)	12 Months Ended
	Sep. 30, 2017 USD ($) ft²	Sep. 30, 2016 USD ($) ft²	Sep. 30, 2015 USD ($)
Other Commitments [Line Items]
Facility rent expense	$ 1,554,000	$ 926,000	$ 831,000
Provision for recorded liabilities	0
Future commitments	11,400,000
Technology License Commitments
Other Commitments [Line Items]
Milestone payments	0	$ 3,000,000	$ 1,000,000
Corporate Headquarters In Pasadena
Other Commitments [Line Items]
Rental expense	$ 27,000
Operating lease expiration month and year	2019-09
Percentage of increase in annual rental cost	3.00%
Office space leases, in square feet \| ft²		8,500
Research Facility in Madison
Other Commitments [Line Items]
Operating lease expiration month and year	2026-09
Office space leases, in square feet \| ft²	60,000
Primary tenant improvement allowance accounted for as deferred rent	$ 2,100,000
Promissory note payable	2,700,000
Rental expense including common area maintenance and real estate taxes and payments of principal and interest on the note payable	$ 182,200
Percentage of increase in annual rental cost excluding principal and interest on the note payable	2.50%
Promissory notes payable amortized term	10 years
Promissory note commencement date	Oct. 01, 2016
Promissory note interest rate	7.10%
Research Facility in Middleton
Other Commitments [Line Items]
Rental expense	$ 18,000
Operating lease expiration month and year	2016-12

Commitments and Contingencies - Future Minimum Lease Payments Under Operating Leases (Detail)	Sep. 30, 2017 USD ($)
Leases [Abstract]
2018	$ 1,531,234
2019	1,435,409
2020	1,044,431
2021	1,070,496
2022	1,097,168
2023 and thereafter	4,669,328
Total	$ 10,848,066

Commitments and Contingencies - Future Principal Payments Under Note Payable (Detail)	Sep. 30, 2017 USD ($)
Long Term Notes Payable [Abstract]
2018	$ 208,506
2019	223,820
2020	240,258
2021	257,903
2022	276,845
2023 and thereafter	1,326,192
Total	$ 2,533,524

Stock-Based Compensation - Additional Information (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options Outstanding	5,549,543	6,691,200
Number of Options Outstanding, Granted	638,000
Stock-based compensation expense	$ 7,891,595	$ 11,595,816	$ 10,232,897
Restricted stock unit vesting, description	At vesting, each RSU will be exchanged for one share of the Company’s Common Stock. Restricted stock unit awards generally vest subject to the satisfaction of service requirements or the satisfaction of both service requirements and achievement of certain performance targets.
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 4,524,833	6,361,396	4,760,831
Grant date fair value of the options granted	849,816	6,426,207	7,338,395
Intrinsic value of options exercised	35,512	142,690	128,391
Unrecognized pre-tax compensation expense	$ 4,627,267
Weighted average period to recognize pre-tax compensation expense	1 year 9 months 19 days
Dividend yield	0.00%
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of restricted stock units outstanding, granted	2,623,000
Stock-based compensation expense	$ 3,366,762	$ 5,234,420	$ 4,489,931
Weighted average period to recognize pre-tax compensation expense	1 year 1 month 7 days
Unrecognized pre-tax compensation expense	$ 2,448,449
2004 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance	2,119,286
2004 Equity Incentive Plan \| Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options Outstanding	2,119,286
Number of Options Outstanding, Granted	0
2004 Equity Incentive Plan \| Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of restricted stock units outstanding, granted	0
2013 Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance	6,329,079
2013 Incentive Plan \| Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options Outstanding	2,986,207
Number of Options Outstanding, Granted	591,000
2013 Incentive Plan \| Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of restricted stock units outstanding	3,098,000
Number of restricted stock units outstanding, granted	2,623,000
Outside Of Equity Compensation Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance	444,050
Outside Of Equity Compensation Plans \| Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options Outstanding, Granted	47,000
Outside Of Equity Compensation Plans \| Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of restricted stock units outstanding	10,000
Number of restricted stock units outstanding, granted	0

Stock-Based Compensation - Summarize Information about Stock Options (Detail)	12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options Outstanding, beginning balance \| shares	6,691,200
Number of Options Outstanding, Granted \| shares	638,000
Number of Options Outstanding, Cancelled \| shares	(1,643,927)
Number of Options Outstanding, Exercised \| shares	(135,730)
Number of Options Outstanding, ending balance \| shares	5,549,543
Number of Options Outstanding, Exercisable \| shares	4,065,611
Weighted-Average Exercise Price Per Share, beginning balance \| $ / shares	$ 6.56
Weighted-Average Exercise Price Per Share, Granted \| $ / shares	1.95
Weighted-Average Exercise Price Per Share, Cancelled \| $ / shares	7.02
Weighted-Average Exercise Price Per Share, Exercised \| $ / shares	2.01
Weighted-Average Exercise Price Per Share, ending balance \| $ / shares	6.00
Weighted-Average Exercise Price Per Share, Exercisable \| $ / shares	$ 6.32
Weighted-Average Remaining Contractual Term	6 years 6 months
Weighted-Average Remaining Contractual Term, Exercisable	5 years 9 months 19 days
Aggregate Intrinsic Value \| $	$ 3,390,029
Aggregate Intrinsic Value, Exercisable \| $	$ 1,872,018

Stock-Based Compensation - Assumptions Used to Value Stock Options (Detail) - $ / shares	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	1.34%	1.05%	1.46%
Risk-free interest rate, maximum	2.31%	1.89%	1.89%
Volatility	79.00%	89.00%	75.00%
Expected life (in years)	5 years 10 months 6 days	6 years 3 months
Weighted average grant date fair value per share of options granted	$ 1.33	$ 4.58	$ 4.24
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected life (in years)			6 years
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected life (in years)			6 years 3 months

Stock-Based Compensation - Summary of Restricted Stock Activity (Detail) - Restricted Stock Units (RSUs)	12 Months Ended
	Sep. 30, 2017 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of RSUs, Unvested, beginning of period \| shares	1,356,667
Number of RSUs, Granted \| shares	2,623,000
Number of RSUs, Vested \| shares	(786,667)
Number of RSUs, Forfeited \| shares	(85,000)
Number of RSUs, Unvested, End of period \| shares	3,108,000
Weighted-Average Grant Date Fair Value, beginning balance \| $ / shares	$ 6.72
Weighted-Average Grant Date Fair Value, Granted \| $ / shares	1.59
Weighted-Average Grant Date Fair Value, Vested \| $ / shares	7.03
Weighted-Average Grant Date Fair Value, Forfeited \| $ / shares	1.55
Weighted-Average Grant Date Fair Value, ending balance \| $ / shares	$ 2.45

Fair Value Measurements - Fair Value Measurements for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Sep. 30, 2017	Sep. 30, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liabilities	$ 695,114	$ 1,602,626
Contingent Consideration	0	0
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	24,838,567	85,366,448
Short-term investments	40,769,539
Derivative liabilities	695,114	1,602,626
Fair Value, Measurements, Recurring \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	24,838,567	85,366,448
Short-term investments	40,769,539
Fair Value, Measurements, Recurring \| Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liabilities	$ 695,114	$ 1,602,626

Fair Value Measurements - Additional Information (Detail)	12 Months Ended
	Sep. 30, 2017 USD ($) shares	Sep. 30, 2016 USD ($) shares	Sep. 30, 2015 USD ($)	Sep. 30, 2011 shares	Jan. 31, 2013 shares	Dec. 31, 2012 shares	Jun. 30, 2010 shares
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Warrants outstanding	432,284
Derivative liability remaining \| $	$ 695,114	$ 1,602,626
Contingent consideration obligations \| $	0	$ 0
Exchange rights \| Ablaris Therapeutics
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Warrants outstanding		500,000
Non-cash gain (loss) from change in fair value of the derivative liability \| $	$ 19,602	$ (7,950)	$ 184,555
Duration of exchange rights	7 years
Exchange right convertible conversion ratio	0.01
Number of exchange rights exercised	500,000			675,000
Derivative liability remaining \| $	$ 0
2010 Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Warrants issued to acquire Common Stock							329,649
Warrants cancelled upon expiration		24,324
2012 Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Warrants issued to acquire Common Stock						912,543
Warrants outstanding	265,161
2013 Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Warrants issued to acquire Common Stock					833,530
Warrants outstanding	12,123
Warrant
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Non-cash gain (loss) from change in fair value of the derivative liability \| $	$ 870,760	$ (293,072)	$ 2,684,712

Fair Value Measurements - Assumptions Used in Valuing Derivative Liabilities (Detail)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
2012 Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Risk-free interest rate	1.07%	0.68%	0.60%
Expected life	2 months 13 days	1 year 2 months 13 days	2 years 2 months 13 days
Volatility	79.00%	89.00%	75.00%
2013 Warrants
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Risk-free interest rate	1.07%	0.68%	0.60%
Expected life	3 months 19 days	1 year 3 months 19 days	2 years 3 months 19 days
Volatility	79.00%	89.00%	75.00%

Fair Value Measurements - Reconciliation of Derivative Liability (Detail) - Warrant - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Value, Beginning balance	$ 1,565,874
Change in value	(870,760)	$ 293,072	$ (2,684,712)
Value, Ending balance	$ 695,114	$ 1,565,874

Income Taxes - Components of the Net Deferred Tax (Liability) and Asset (Detail) - USD ($)	Sep. 30, 2017	Sep. 30, 2016
Deferred tax assets:
Accrued compensation	$ 1,337,619	$ 1,691,050
Stock compensation	5,670,468	7,224,958
Capitalized research and development	2,196,542	2,957,528
Fixed Assets	203,477
Net operating losses	132,119,837	125,120,957
Intangible Assets	5,358,700	5,544,091
Deferred Revenue	1,068,206
Deferred Rent	865,675
Capital Loss	1,020,162
Total deferred tax assets	149,840,686	142,538,584
Valuation allowance	(136,625,436)	(128,695,371)
Deferred tax liabilities:
State taxes	(13,215,250)	(13,804,268)
Fixed assets		(38,945)
Total deferred tax liability	$ (13,215,250)	$ (13,843,213)

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Valuation allowance against deferred tax assets	100.00%
Gross federal net operating loss carry forwards		$ 260.7
Gross federal estimated net operating loss carry forwards	$ 24.5
Gross state net operating loss carry forwards		$ 398.3
Gross state estimated net operating loss carry forwards	$ 24.2

Income Taxes - Provisions for Income Taxes (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
State:
Current	$ 2,400	$ 2,400
Total State	2,400	2,400
Provision from income taxes	$ 2,400	$ 2,400	$ 2,400

Income Taxes - Summary of Effective Income Tax Rate Reconciliation (Detail)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
At U.S. federal statutory rate	34.00%	34.00%
State taxes, net of federal effect	4.90%	9.00%
Stock compensation	(5.80%)	(1.10%)
Mark-to-market adjustments	0.90%	(0.10%)
Valuation allowance	(23.10%)	(41.10%)
True-up on deferred taxes	(3.00%)
State blended rate change	(5.70%)
Other	(2.20%)	(0.70%)
Effective income tax rate	0.00%	0.00%

Employee Benefit Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Employee benefits costs	$ 426,470	$ 476,835	$ 407,603
Employee Contributions up to 3%
Defined Benefit Plan Disclosure [Line Items]
Discretionary match percentage	100.00%
Percentage of compensation	3.00%
Employee Contributions Next 2%
Defined Benefit Plan Disclosure [Line Items]
Discretionary match percentage	50.00%
Percentage of compensation	2.00%

Unaudited Quarterly Financial Data - Summary of Unaudited Quarterly Financial Data (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 8,713,785	$ 9,342,498	$ 8,985,930	$ 4,365,496	$ 31,250	$ 39,583	$ 43,750	$ 43,750	$ 31,407,709	$ 158,333	$ 382,000
Operating Losses	(10,250,216)	(5,714,164)	(6,129,642)	(14,901,887)	(21,795,114)	(19,341,487)	(21,264,855)	(19,341,270)	(36,995,909)	(81,742,726)	(95,973,976)
Net loss	(10,731,889)	(5,519,741)	(6,042,557)	(12,086,108)	(22,221,985)	(19,420,743)	(20,815,860)	(19,264,414)	$ (34,380,295)	$ (81,723,002)	$ (91,940,882)
Net Loss Attributable to Arrowhead	$ (10,731,889)	$ (5,519,741)	$ (6,042,557)	$ (12,086,108)	$ (22,221,985)	$ (19,420,743)	$ (20,815,860)	$ (19,264,414)
Loss per share (Basic and Diluted)	$ (0.14)	$ (0.07)	$ (0.08)	$ (0.17)	$ (0.34)	$ (0.32)	$ (0.35)	$ (0.32)	$ (0.47)	$ (1.34)	$ (1.60)