View Filing Data

Document and Entity Information - shares	9 Months Ended
Jun. 30, 2018	Aug. 06, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q3
Trading Symbol	ARWR
Entity Registrant Name	ARROWHEAD PHARMACEUTICALS, INC.
Entity Central Index Key	0000879407
Current Fiscal Year End Date	--09-30
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		88,023,149

Consolidated Balance Sheets - USD ($)	Jun. 30, 2018	Sep. 30, 2017
CURRENT ASSETS
Cash and cash equivalents	$ 27,995,386	$ 24,838,567
Accounts receivable	14,975	67,797
Prepaid expenses	1,095,977	867,363
Other current assets	279,049	1,359,638
Short term investments	32,484,708	40,769,539
TOTAL CURRENT ASSETS	61,870,095	67,902,904
Long term investments	17,672,658
Property and equipment, net	14,029,239	15,513,019
Intangible assets, net	19,189,117	20,464,439
Other assets	141,918	141,918
TOTAL ASSETS	112,903,027	104,022,280
CURRENT LIABILITIES
Accounts payable	2,165,731	4,076,514
Accrued expenses	2,163,533	4,564,507
Accrued payroll and benefits	1,147,780	3,399,679
Deferred rent	440,580	440,580
Deferred revenue	943,200	5,269,741
Derivative liabilities		695,114
Note Payable	219,889	208,506
Other current liabilities	46,407	46,407
TOTAL CURRENT LIABILITIES	7,127,120	18,701,048
LONG-TERM LIABILITIES
Deferred rent, net of current portion	1,657,899	1,929,052
Note Payable, net of current portion	2,158,649	2,325,018
Other non-current liabilities	200,000	200,000
TOTAL LONG-TERM LIABILITIES	4,016,548	4,454,070
Commitments and contingencies (Note 7)
Arrowhead Pharmaceuticals, Inc. stockholders' equity:
Common stock, $0.001 par value; 145,000,000 shares authorized; 87,854,651 and 74,785,426 shares issued and outstanding as of June 30, 2018 and September 30, 2017, respectively	180,224	167,155
Additional paid-in capital	578,642,206	514,037,301
Accumulated other comprehensive income (loss)	(3,338)	33,232
Accumulated deficit	(476,504,545)	(432,815,338)
Total Arrowhead Pharmaceuticals, Inc. stockholders' equity	102,314,547	81,422,350
Noncontrolling interest	(555,188)	(555,188)
TOTAL STOCKHOLDERS’ EQUITY	101,759,359	80,867,162
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	$ 112,903,027	$ 104,022,280

Consolidated Statements of Operations and Comprehensive Loss (unaudited) - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
REVENUE	$ 727,375	$ 9,342,498	$ 4,887,321	$ 22,693,923
Type of Revenue [Extensible List]	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember
OPERATING EXPENSES
Research and development	$ 12,052,653	$ 11,136,741	$ 36,974,625	$ 37,363,207
General and administrative expenses	4,594,441	3,919,921	12,679,822	12,076,412
TOTAL OPERATING EXPENSES	16,647,094	15,056,662	49,654,447	49,439,619
OPERATING LOSS	(15,919,719)	(5,714,164)	(44,767,126)	(26,745,696)
OTHER INCOME (EXPENSE)
Interest income (expense), net	313,702	131,358	645,778	265,250
Change in value of derivatives		63,065	432,141	1,519,513
Other income (expense)				1,312,524
TOTAL OTHER INCOME (EXPENSE)	313,702	194,423	1,077,919	3,097,287
LOSS BEFORE INCOME TAXES	(15,606,017)	(5,519,741)	(43,689,207)	(23,648,409)
NET LOSS	$ (15,606,017)	$ (5,519,741)	$ (43,689,207)	$ (23,648,409)
NET LOSS PER SHARE - BASIC & DILUTED	$ (0.18)	$ (0.07)	$ (0.53)	$ (0.32)
Weighted average shares outstanding - basic and diluted	87,634,435	74,772,103	82,149,381	73,603,852
OTHER COMPREHENSIVE INCOME (LOSS), NET OF TAX:
Foreign Currency Translation Adjustments	$ (28,603)	$ 12,309	$ (36,570)	$ (25,835)
COMPREHENSIVE LOSS	$ (15,634,620)	$ (5,507,432)	$ (43,725,777)	$ (23,674,244)

Consolidated Statement of Stockholders' Equity (unaudited) - 9 months ended Jun. 30, 2018 - USD ($)	Total	Common stock issued for cash at $5.25 per share	Common Stock	Common Stock Common stock issued for cash at $5.25 per share	Additional Paid-In Capital	Additional Paid-In Capital Common stock issued for cash at $5.25 per share	Accumulated Other Comprehensive Income (loss)	Accumulated Deficit	Non-controlling Interest
Beginning Balance, Amount at Sep. 30, 2017	$ 80,867,162		$ 167,155		$ 514,037,301		$ 33,232	$ (432,815,338)	$ (555,188)
Beginning Balance, Shares at Sep. 30, 2017			74,785,426
Stock-based compensation	5,818,119				5,818,119
Exercise of stock options, Amount	$ 1,032,058		$ 283		1,031,775
Exercise of stock options, Shares	283,627		283,627
Exercise of warrants, Amount	$ 1,237,430		$ 289		1,237,141
Exercise of warrants, Shares			288,473
Common stock- Restricted Stock Units vesting, Amount	(54,668)		$ 997		(55,665)
Common stock- Restricted Stock Units vesting, Shares			997,125
Stock issuances		$ 56,585,035		$ 11,500		$ 56,573,535
Stock issuances, Shares				11,500,000
Foreign currency translation adjustments	(36,570)						(36,570)
Net loss for the nine months ended June 30, 2018	(43,689,207)							(43,689,207)
Ending Balance, Amount at Jun. 30, 2018	$ 101,759,359		$ 180,224		$ 578,642,206		$ (3,338)	$ (476,504,545)	$ (555,188)
Ending Balance, Shares at Jun. 30, 2018			87,854,651

Consolidated Statements of Cash Flows (unaudited) - USD ($)	9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (43,689,207)	$ (23,648,409)
Change in value of derivatives	(432,141)	(1,519,513)
Stock-based compensation	5,818,119	5,881,901
Depreciation and amortization	3,470,880	3,535,915
Amortization/(accretion) of note premiums	582,373	(140,041)
Changes in operating assets and liabilities:
Accounts receivable	52,822	(862,313)
Prepaid expenses and Other Current Assets	856,253	3,176,551
Deferred revenue	(4,326,540)	8,796,246
Accounts payable	(1,910,782)	(4,353,087)
Accrued expenses	(4,652,873)	(5,337,827)
Other	(312,003)	(264,921)
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	(44,543,099)	(14,735,498)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(711,779)	(7,810,754)
Purchases of marketable securities	(47,174,620)	(39,904,676)
Proceeds from sale of marketable securities	37,204,420	3,289,963
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	(10,681,979)	(44,425,467)
CASH FLOWS FROM FINANCING ACTIVITIES:
Principal payments on notes payable	(154,986)	(97,145)
Payments of taxes for net share settled restricted stock unit issuances	(54,667)	(417,140)
Proceeds from the exercises of warrants and stock options	2,006,515	272,818
Proceeds from the issuance of common stock	56,585,035	12,419,119
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	58,381,897	12,177,652
NET INCREASE (DECREASE) IN CASH	3,156,819	(46,983,313)
CASH AT BEGINNING OF PERIOD	24,838,567	85,366,448
CASH AT END OF PERIOD	27,995,386	38,383,135
Supplementary disclosures:
Interest Paid	(131,429)	(142,033)
Income Tax Credits Refunded		3,635,016
Income Tax Paid	$ (2,400)	$ (2,400)

Organization and Significant Accounting Policies	9 Months Ended
Organization and Significant Accounting Policies	Jun. 30, 2018
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-ENaC for cystic fibrosis, ARO-HIF2 for renal cell carcinoma, and ARO-AMG1 for an undisclosed genetically validated cardiovascular target under a license and collaboration agreement with Amgen, Inc., a Delaware corporation (“Amgen”). AMG 890 (ARO-LPA) for cardiovascular disease was out-licensed to Amgen in 2016. With regard to key recent developments, during fiscal 2018, the Company filed Clinical Trial Applications (CTAs) for ARO-AAT and ARO-HBV to begin a phase 1 clinical study and a phase 1 / 2 clinical study for each program, respectively, and dosing is now underway in both studies. Additionally, on January 22, 2018, the Company sold 11,500,000 shares of Common Stock in a fully underwritten public offering, at a public offering price of $5.25 per share. Net proceeds to the Company were approximately $56.6 million after deducting underwriting commissions and discounts and other offering expenses payable by the Company. 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 June 30, 2018, the Company had $28.0 million in cash, $32.5 million in short-term investments, and $17.7 in long-term investments to fund operations. During the nine months ended June 30, 2018, the Company’s cash and investments balance increased by $12.5 million, which was primarily the result of the $56.6 million net proceeds received from the public offering discussed above partially offset by cash outflows of $44.5 million related to operating activities. On January 22, 2018, the Company sold 11,500,000 shares of Common Stock in a fully underwritten public offering, at a public offering price of $5.25 per share. Net proceeds to the Company were approximately $56.6 million after deducting underwriting commissions and discounts and other offering expenses payable by the Company. The Company believes its current financial resources are sufficient to fund its operations through at least the next twelve months. 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. In the opinion of management, all adjustments, consisting of normal recurring accruals, considered necessary for a fair presentation have been included. 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. These condensed consolidated financial statements should be read in conjunction with the Company’s consolidated financial statements and the notes thereto contained in our Annual Report on Form 10-K for the year ended September 30, 2017. 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 June 30, 2018 and September 30, 2017. 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 consist of 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 debt 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 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 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 three and nine months ended June 30, 2018 and 2017, respectively, 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 license agreements. 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 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 June 30, 2018 and September 30, 2017. 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, research grants 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 generally referred to as 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 is determined to be a separate unit of accounting from the other undelivered performance obligations. The undelivered performance obligations typically include manufacturing or development services or research and/or steering committee services. 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, 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 three and nine months ended June 30, 2018 and 2017 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, with consideration given to the probability of the performance condition being achieved. 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. 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. In April 2016, the FASB issued an amendment to ASU No. 2014-09 with update ASU 2016-10 which provided more specific guidance around the identification of performance obligations and licensing arrangements. 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	9 Months Ended
Collaboration and License Agreements – Amgen, Inc	Jun. 30, 2018
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 “AMG 890 (ARO-LPA) Agreement”), Amgen has received a worldwide, exclusive license to Arrowhead’s novel, RNAi ARO-LPA 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 the Effective Date, as defined in the ARO-AMG1 Agreement. 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 AMG 890 (ARO-LPA) 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 Company received the upfront payment of $5 million due under this agreement in September 2016. The upfront $5 million payment was recorded as Deferred Revenue, and $0.6 million and $2.0 million of this was amortized into Revenue during the three and nine months ended June 30, 2018, respectively. During the three and nine months ended June 30, 2017, $0.6 million and $2.0 million of this upfront $5 million payment was amortized in Revenue, respectively. Of the upfront $5 million payment, approximately $0.6 million remained as Deferred Revenue as of June 30, 2018. Under the terms of the AMG 890 (ARO-LPA) Agreement, the Company has granted a worldwide, exclusive license to AMG 890 (ARO-LPA). 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 AMG 890 (ARO-LPA) 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 AMG 890 (ARO-LPA) 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 recognized revenue on a straight-line basis from November 18, 2016 (the Hart-Scott-Rodino clearance date), through October 31, 2017, which was the date where the significant development and manufacturing related deliverables were completed. The Company received the upfront payment of $30 million due under the AMG 890 (ARO-LPA) Agreement in November 2016. The upfront $30 million payment was recorded as Deferred Revenue, and $2.7 million of this was amortized into Revenue during the three months ended December 31, 2017. The upfront $30 million payment has been fully recognized, and $0 remains in Deferred Revenue as of June 30, 2018. During the three and nine months ended June 30, 2017, $7.9 million and $19.4 million of the upfront $30 million payment was amortized into Revenue, respectively. On August 1, 2018, the Company announced that it had earned a $10 million milestone payment from Amgen following the administration of the first dose of AMG 890 (ARO-LPA) in a phase 1 clinical study. The Company will recognize this payment as revenue in its entirety during the fourth fiscal quarter of 2018. The Company also entered into a separate services agreement and separate statements of work with Amgen to provide certain services related to process development, manufacturing, materials supply, discovery studies, and other consulting services related to AMG 890 (ARO-LPA) and ARO-AMG1. During the three and nine months ended June 30, 2018, these work orders generated approximately $0.1 million and $0.3 million of Revenue, respectively. During the three and nine months ended June 30, 2017, these work orders generated approximately $0.8 million and $1.4 million of Revenue, respectively.

Investments

9 Months Ended

Jun. 30, 2018

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 June 30, 2018 consisted of corporate bonds with maturities remaining of less than 15 months. 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 June 30, 2018, all investments were classified as held-to-maturity securities.

The following tables summarize the Company’s short-term and long-term investments as of June 30, 2018, and September 30, 2017.

	As of June 30, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Commercial notes (due within one year)	$	32,484,708	$	—	$	(237,007	)	$	32,247,701
Commercial notes (due after one year through two years)	$	17,672,658		—	$	(98,327	)	$	17,574,331
Total	$	50,157,366	$	—	$	(335,334	)	$	49,822,032

	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
Commercial notes (due after one year through two years)	$	—		—	$	—		$	—
Total	$	40,769,539	$	—	$	(334,755	)	$	40,434,784

Commitments and Contingencies

9 Months Ended

Jun. 30, 2018

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. Monthly rental payments 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 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.

Facility rent expense for the three months ended June 30, 2018 and 2017 was $353,400 and $328,600, respectively. Facility rent expense for the nine months ended June 30, 2018 and 2017 was $1,041,400 and $1,126,500, respectively.

As of June 30, 2018, future minimum lease payments due in fiscal years under operating leases are as follows:

2018 (remainder of)	$	384,250
2019		1,435,409
2020		1,044,431
2021		1,070,496
2022		1,097,168
2023 and thereafter		4,669,328
Total	$	9,701,082

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 bears interest at a rate of 7.1% and is payable in equal monthly installments of principal and interest.

As of June 30, 2018, future principal payments due in fiscal years under the note payable are as follows:

2018 (remainder of)	$	53,520
2019		223,820
2020		240,258
2021		257,903
2022		276,845
2023 and thereafter		1,326,192
Total	$	2,378,538

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 was filed 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. Following the dismissal of the consolidated class action, the parties for the Weisman and Johnson actions jointly stipulated to dismiss the actions, with the parties bearing their own fees and costs. The parties to the Bernstein and consolidated derivative action agreed to stay the matters pending the resolution of the Ninth Circuit appeal of the dismissal of the consolidated class action. On February 15, 2018, the Ninth Circuit issued a memorandum affirming the district court’s dismissal of all claims. Plaintiffs in the consolidated derivative action voluntarily dismissed their case. The parties to the Bernstein action filed a stipulation to continue the stay of the action pending resolution of the Ninth Circuit appeal in Meller v. Arrowhead Pharmaceuticals, Inc., Case No. 2:16-cv-08505 (C.D. Cal.). 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.

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 December 21, 2017, the federal district court dismissed the consolidated class action with prejudice. On December 27, 2017 the plaintiffs appealed the dismissal to the United States Court of Appeals for the Ninth Circuit. The Lucas and Singh actions are stayed pending resolution of the Ninth Circuit appeal. 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 clinical studies. As of June 30, 2018, these future commitments were estimated at approximately $19.2 million, of which approximately $5.0 million is expected to be incurred in fiscal 2018, and $14.2 is expected to be incurred beyond 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 three and nine months ended June 30, 2018 and 2017, the Company did not reach any milestones requiring milestone payments. 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

9 Months Ended

Jun. 30, 2018

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 June 30, 2018, 1,918,338 and 6,733,430 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 awards to employees, consultants and others. No further grants may be made under the 2004 Equity Incentive Plan. As of June 30, 2018, there were options granted and outstanding to purchase 1,918,338 and 3,204,521 shares of Common Stock under the 2004 Equity Incentive Plan and the 2013 Incentive Plan, respectively, and there were 3,295,665 restricted stock units granted and outstanding under the 2013 Incentive Plan. Also, as of June 30, 2018, there were 648,700 shares reserved for options and 2,500 restricted stock units issued as inducement grants to new employees outside of equity compensation plans. During the three months ended June 30, 2018, no options or restricted stock units were granted under the 2004 Equity Incentive Plan, no options or restricted stock units were granted under the 2013 Incentive Plan, and 79,000 options and no restricted stock units were granted as inducement awards to new employees outside of equity incentive plans. During the nine months ended June 30, 2018, no options or restricted stock units were granted under the 2004 Equity Incentive Plan, 467,000 options and 1,243,000 restricted stock units were granted under the 2013 Incentive Plan, and 272,000 options and 2,500 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, 2017		5,549,543	$	6.00
Granted		739,000		4.71
Cancelled		(233,357)		6.97
Exercised		(283,627)		3.64
Balance At June 30, 2018		5,771,559	$	5.92		6.2 years	$	45,386,696
Exercisable At June 30, 2018		4,174,006	$	6.53		5.3 years	$	30,543,795

Stock-based compensation expense related to stock options for the three months ended June 30, 2018 and 2017 was $744,396 and $1,021,653, respectively. Stock-based compensation expense related to stock options for the nine months ended June 30, 2018 and 2017 was $2,503,060 and $3,558,082, 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 three months ended June 30, 2018 and 2017 was $640,687 and $70,937, respectively. The grant date fair value of the options granted by the Company for the nine months ended June 30, 2018 and 2017 was $2,933,593 and $849,816, respectively.

The intrinsic value of the options exercised during the three months ended June 30, 2018 and 2017 was $1,486,390 and $0, respectively. The intrinsic value of the options exercised during the nine months ended June 30, 2018 and 2017 was $1,837,064 and $35,512, respectively.

As of June 30, 2018, the pre-tax compensation expense for all outstanding unvested stock options in the amount of approximately $4,690,290 will be recognized in the Company’s results of operations over a weighted average period of 2.5 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:

	Nine Months Ended June 30,
	2018	2017
Dividend yield	—	—
Risk-free interest rate	2.1 – 2.8%	1.3 – 2.3%
Volatility	110%	79%
Expected life (in years)	6.25	5.75 - 6.25
Weighted average grant date fair value per share of options granted	$3.97	$1.33

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 three months ended June 30, 2018, the Company issued no RSUs under the 2013 Incentive Plan and no RSUs outside of the equity incentive plans. During the nine months ended June 30, 2018, the Company issued 1,243,000 RSUs under the 2013 Incentive Plan and 2,500 RSUs as an inducement award to a new employee outside of the equity incentive plans. At vesting, each outstanding RSU will be exchanged for one share of the Company’s Common Stock. RSU recipients may elect to net share settle upon vesting, in which case the Company pays the employee’s income taxes due upon vesting and withholds a number of shares of Common Stock of equal value. RSU 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 RSUs:

	Number of RSUs			Weighted- Average Grant Date Fair Value
Unvested at September 30, 2017		3,108,000		$	2.45
Granted		1,245,500			3.68
Vested		(1,005,333	)		2.35
Forfeited		(50,000	)		1.55
Unvested at June 30, 2018		3,298,167		$	2.96

During the three months ended June 30, 2018 and 2017, the Company recorded $1,524,369 and $691,575 of expense related to RSUs, respectively. During the nine months ended June 30, 2018 and 2017, the Company recorded $3,315,059 and $2,323,819 of expense related to RSUs, respectively. Such expense is included in stock-based compensation expense in the Company’s Consolidated Statement of Operations and Comprehensive Loss.

For RSUs, the grant date fair value of the award is based on the Company’s closing stock price at the grant date, with consideration given to the probability of achieving performance conditions for performance based awards.

As of June 30, 2018, the pre-tax compensation expense for all unvested RSUs in the amount of approximately $2,950,140 will be recognized in the Company’s results of operations over a weighted average period of 2.6 years.

Fair Value Measurements

9 Months Ended

Jun. 30, 2018

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 June 30, 2018 and September 30, 2017 for assets and liabilities measured at fair value on a recurring basis:

June 30, 2018:

	Level 1		Level 2		Level 3		Total
Cash and cash equivalents	$	27,995,386	$	—	$	—	$	27,995,386
Short-term investments		32,247,701		—		—		32,247,701
Long-term investments		17,574,331		—		—		17,574,331
Derivative liabilities		—		—		—		—
Contingent Consideration	$	—	$	—	$	—	$	—

September 30, 2017:

	Level 1		Level 2		Level 3		Total
Cash and cash equivalents	$	24,838,567	$	—	$	—	$	24,838,567
Short-term investments		40,434,784		—		—		40,434,784
Long-term investments		—		—		—		—
Derivative liabilities		—		—		695,114		695,114
Contingent Consideration	$	—	$	—	$	—	$	—

As part of a financing in January 2013, Arrowhead issued warrants to purchase up to 833,530 shares of Common Stock (the “2013 Warrants”) of which 0 warrants were outstanding at June 30, 2018. Further, 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 warrants to exercise 143,811 shares remained unexercised and were cancelled at their expiration during the three months ended December 31, 2017. Each of the Warrants contained a mechanism to adjust the strike price upon the issuance of certain dilutive equity securities. If during the terms of the Warrants, the Company issued 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 were 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 was estimated at the end of each reporting period and the change in the fair value of the Warrants was 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 three months ended June 30, 2018 and 2017, the Company recorded a non-cash gain/(loss) from the change in fair value of the derivative liability of $0 and $61,915, respectively. During the nine months ended June 30, 2018 and 2017, the Company recorded a non-cash gain/(loss) from the change in fair value of the derivative liability of $432,141 and $1,490,863, respectively.

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

Value at September 30, 2017	$	695,114
Issuance of instruments		—
Change in value		(432,141	)
Net settlements		(262,973	)
Value at June 30, 2018	$	—

The derivative assets/liabilities were 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 was 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 June 30, 2018 and September 30, 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

Organization and Significant Accounting Policies (Policies)	9 Months Ended
	Jun. 30, 2018
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. In the opinion of management, all adjustments, consisting of normal recurring accruals, considered necessary for a fair presentation have been included. 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. These condensed consolidated financial statements should be read in conjunction with the Company’s consolidated financial statements and the notes thereto contained in our Annual Report on Form 10-K for the year ended September 30, 2017.
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 June 30, 2018 and September 30, 2017.
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 consist of 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 debt 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 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 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 three and nine months ended June 30, 2018 and 2017, respectively, 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 license agreements. 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 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 June 30, 2018 and September 30, 2017.
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, research grants 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 generally referred to as 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 is determined to be a separate unit of accounting from the other undelivered performance obligations. The undelivered performance obligations typically include manufacturing or development services or research and/or steering committee services. 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, 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 three and nine months ended June 30, 2018 and 2017 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, with consideration given to the probability of the performance condition being achieved. 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.
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. In April 2016, the FASB issued an amendment to ASU No. 2014-09 with update ASU 2016-10 which provided more specific guidance around the identification of performance obligations and licensing arrangements. 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.

Stock-Based Compensation (Tables)

9 Months Ended

Jun. 30, 2018

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, 2017		5,549,543	$	6.00
Granted		739,000		4.71
Cancelled		(233,357)		6.97
Exercised		(283,627)		3.64
Balance At June 30, 2018		5,771,559	$	5.92		6.2 years	$	45,386,696
Exercisable At June 30, 2018		4,174,006	$	6.53		5.3 years	$	30,543,795

Assumptions Used to Value Stock Options

The assumptions used to value stock options are as follows:

	Nine Months Ended June 30,
	2018	2017
Dividend yield	—	—
Risk-free interest rate	2.1 – 2.8%	1.3 – 2.3%
Volatility	110%	79%
Expected life (in years)	6.25	5.75 - 6.25
Weighted average grant date fair value per share of options granted	$3.97	$1.33

Summary of Share Activity Related to RSUs

The following table summarizes the activity of the Company’s RSUs:

	Number of RSUs			Weighted- Average Grant Date Fair Value
Unvested at September 30, 2017		3,108,000		$	2.45
Granted		1,245,500			3.68
Vested		(1,005,333	)		2.35
Forfeited		(50,000	)		1.55
Unvested at June 30, 2018		3,298,167		$	2.96

Fair Value Measurements (Tables)

9 Months Ended

Jun. 30, 2018

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 June 30, 2018 and September 30, 2017 for assets and liabilities measured at fair value on a recurring basis:

June 30, 2018:

	Level 1		Level 2		Level 3		Total
Cash and cash equivalents	$	27,995,386	$	—	$	—	$	27,995,386
Short-term investments		32,247,701		—		—		32,247,701
Long-term investments		17,574,331		—		—		17,574,331
Derivative liabilities		—		—		—		—
Contingent Consideration	$	—	$	—	$	—	$	—

September 30, 2017:

	Level 1		Level 2		Level 3		Total
Cash and cash equivalents	$	24,838,567	$	—	$	—	$	24,838,567
Short-term investments		40,434,784		—		—		40,434,784
Long-term investments		—		—		—		—
Derivative liabilities		—		—		695,114		695,114
Contingent Consideration	$	—	$	—	$	—	$	—

Warrant

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

Reconciliation of Derivative Liability

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

Value at September 30, 2017	$	695,114
Issuance of instruments		—
Change in value		(432,141	)
Net settlements		(262,973	)
Value at June 30, 2018	$	—

Organization and Significant Accounting Policies - Additional Information (Detail) - USD ($)		9 Months Ended
	Jan. 22, 2018	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016
Organization And Significant Accounting Policies [Line Items]
Public offering price per share		$ 5.25
Cash and cash equivalents		$ 27,995,386	$ 38,383,135	$ 24,838,567	$ 85,366,448
Short term investments		32,484,708		40,769,539
Long term investments		17,672,658
Net increase (decrease) in cash and investments		12,500,000
Net proceeds received from public offering		56,585,035	12,419,119
Cash outflows related to operating activities		(44,543,099)	$ (14,735,498)
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
Underwritten Public Offering
Organization And Significant Accounting Policies [Line Items]
Common stock shares sold	11,500,000
Public offering price per share	$ 5.25
Net proceeds, after deducting underwriting commissions and discounts and offering expenses payable	$ 56,600,000

Collaboration and License Agreements - Amgen, Inc - Additional Information (Detail)		1 Months Ended		3 Months Ended			9 Months Ended
	Sep. 28, 2016 USD ($) Agreement $ / shares shares	Nov. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) $ / shares	Jun. 30, 2017 USD ($)	Aug. 01, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Stock issued, price per share \| $ / shares				$ 5.25			$ 5.25
Proceeds from the issuance of common stock							$ 56,585,035	$ 12,419,119
Revenue generated from work orders				$ 727,375		$ 9,342,498	$ 4,887,321	$ 22,693,923
Type of Revenue [Extensible List]				arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember		arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember
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
Cash received as due under collaboration agreement			$ 5,000,000
Deferred revenue			$ 5,000,000	$ 600,000			$ 600,000
Deferred revenue amortized into revenue				600,000		$ 600,000	2,000,000	$ 2,000,000
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]
Cash received as due under collaboration agreement	35,000,000
Proceeds from the issuance of common stock	$ 21,500,000
AMG-890 (ARO-LPA) Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash received as due under collaboration agreement		$ 30,000,000
Deferred revenue		$ 30,000,000		0			0
Deferred revenue amortized into revenue					$ 2,700,000	7,900,000		19,400,000
AMG-890 (ARO-LPA) Agreement \| Amgen \| Subsequent Event
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amgen milestone payment									$ 10,000,000
AMG 890 (ARO-LPA) and ARO-AMG1 Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue generated from work orders				$ 100,000		$ 800,000	$ 300,000	$ 1,400,000
Type of Revenue [Extensible List]				us-gaap:LicenseAndServiceMember		us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Common Stock Purchase Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Shares issued \| shares	3,002,793
Stock issued, price per share \| $ / shares	$ 7.16
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

Intangible Assets - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Finite Lived Intangible Assets [Line Items]
Amortization expense	$ 425,107	$ 425,107	$ 1,275,322	$ 1,275,322
Amortization of license agreements remainder of fiscal year 2018	425,108		425,108
Amortization of license agreements in 2019	1,700,429		1,700,429
Amortization of license agreements in 2020	1,700,429		1,700,429
Amortization of license agreements in 2021	1,700,429		1,700,429
Amortization of license agreements in 2022	1,700,429		1,700,429
Amortization of license agreements in 2023	1,700,429		1,700,429
Amortization of license agreements, thereafter	10,261,864		$ 10,261,864
Novartis \| Licensing Agreement
Finite Lived Intangible Assets [Line Items]
Amortization period of intangible assets			21 years
Finite-lived intangible assets, accumulated amortization	494,684		$ 494,684
Novartis \| Patents
Finite Lived Intangible Assets [Line Items]
Amortization period of intangible assets			14 years
Finite-lived intangible assets, accumulated amortization	$ 5,173,413		$ 5,173,413

Commitments and Contingencies - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Jun. 30, 2018 USD ($) ft²	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) ft²	Jun. 30, 2017 USD ($)
Other Commitments [Line Items]
Facility rent expense	$ 353,400	$ 328,600	$ 1,041,400	$ 1,126,500
Provision for recorded liabilities	0		0
Future commitments	19,200,000		19,200,000
Commitments expected to be incurred in fiscal 2018	5,000,000		5,000,000
Commitments expected to be incurred beyond fiscal 2018	14,200,000		14,200,000
Technology License Commitments
Other Commitments [Line Items]
Milestone payments	$ 0	$ 0	$ 0	$ 0
Corporate Headquarters In Pasadena
Other Commitments [Line Items]
Office space leases, in square feet \| ft²	8,500		8,500
Operating lease expiration month and year			2019-09
Rental expense			$ 27,000
Percentage of increase in annual rental cost			3.00%
Research Facility in Madison
Other Commitments [Line Items]
Office space leases, in square feet \| ft²	60,000		60,000
Operating lease expiration month and year			2026-09
Primary tenant improvement allowance accounted for as deferred rent	$ 2,100,000		$ 2,100,000
Promissory note payable	$ 2,700,000		2,700,000
Rental expense including 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%

Stock-Based Compensation - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options Outstanding	5,771,559		5,771,559		5,549,543
Number of Options Outstanding, Granted			739,000
RSU awards vesting, description			At vesting, each outstanding RSU will be exchanged for one share of the Company’s Common Stock. RSU recipients may elect to net share settle upon vesting, in which case the Company pays the employee’s income taxes due upon vesting and withholds a number of shares of Common Stock of equal value. RSU 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	$ 744,396	$ 1,021,653	$ 2,503,060	$ 3,558,082
Grant date fair value of the options granted	640,687	70,937	2,933,593	849,816
Intrinsic value of options exercised	1,486,390	0	1,837,064	35,512
Unrecognized pre-tax compensation expense	4,690,290		$ 4,690,290
Weighted average period to recognize pre-tax compensation expense			2 years 6 months
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			1,245,500
Stock-based compensation expense	1,524,369	$ 691,575	$ 3,315,059	$ 2,323,819
Weighted average period to recognize pre-tax compensation expense			2 years 7 months 6 days
Unrecognized pre-tax compensation expense	$ 2,950,140		$ 2,950,140
2004 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance	1,918,338		1,918,338
2004 Equity Incentive Plan \| Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options Outstanding	1,918,338		1,918,338
Number of Options Outstanding, Granted	0		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		0
2013 Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance	6,733,430		6,733,430
2013 Incentive Plan \| Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options Outstanding	3,204,521		3,204,521
Number of Options Outstanding, Granted	0		467,000
2013 Incentive Plan \| Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of restricted stock units outstanding	3,295,665		3,295,665
Number of restricted stock units outstanding, granted	0		1,243,000
Outside Of Equity Compensation Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserve for issuance	648,700		648,700
Outside Of Equity Compensation Plans \| Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options Outstanding, Granted	79,000		272,000
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	2,500		2,500
Number of restricted stock units outstanding, granted	0		2,500

Stock-Based Compensation - Summarize Information about Stock Options (Detail) - USD ($)	9 Months Ended
	Jun. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options Outstanding, beginning balance	5,549,543
Number of Options Outstanding, Granted	739,000
Number of Options Outstanding, Cancelled	(233,357)
Number of Options Outstanding, Exercised	(283,627)
Number of Options Outstanding, ending balance	5,771,559
Number of Options Outstanding, Exercisable	4,174,006
Weighted-Average Exercise Price Per Share, beginning balance	$ 6.00
Weighted-Average Exercise Price Per Share, Granted	4.71
Weighted-Average Exercise Price Per Share, Cancelled	6.97
Weighted-Average Exercise Price Per Share, Exercised	3.64
Weighted-Average Exercise Price Per Share, ending balance	5.92
Weighted-Average Exercise Price Per Share, Exercisable	$ 6.53
Weighted-Average Remaining Contractual Term	6 years 2 months 12 days
Weighted-Average Remaining Contractual Term, Exercisable	5 years 3 months 18 days
Aggregate Intrinsic Value	$ 45,386,696
Aggregate Intrinsic Value, Exercisable	$ 30,543,795

Consolidated Statement of Stockholders' Equity (unaudited) (Parenthetical)	Jun. 30, 2018 $ / shares
Statement Of Stockholders Equity [Abstract]
Stock issued, price per share	$ 5.25

Stockholders' Equity	9 Months Ended
Stockholders' Equity	Jun. 30, 2018
Equity [Abstract]
Stockholders' Equity	NOTE 6. STOCKHOLDERS’ EQUITY At June 30, 2018, 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 June 30, 2018, 87,854,651 shares of Common Stock were outstanding. At June 30, 2018, 9,302,968 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. On January 22, 2018, the Company sold 11,500,000 shares of Common Stock in a fully underwritten public offering, at a public offering price of $5.25 per share. Net proceeds to the Company were approximately $56.6 million after deducting underwriting commissions and discounts and other offering expenses payable by the Company.

Subsequent Events	9 Months Ended
Subsequent Events	Jun. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	NOTE 10. SUBSEQUENT EVENTS On August 1, 2018, the Company announced that it had earned a $10 million milestone payment from Amgen following the administration of the first dose of AMG 890 (ARO-LPA) in a phase 1 clinical study. The Company will recognize this payment as revenue in its entirety during the fourth fiscal quarter of 2018.

Property and Equipment - Summary of Property and Equipment (Detail) - USD ($)	Jun. 30, 2018	Sep. 30, 2017
Property Plant And Equipment [Abstract]
Computers, office equipment and furniture	$ 600,334	$ 600,334
Research equipment	10,345,664	9,660,960
Software	148,829	132,078
Leasehold improvements	12,208,380	12,208,380
Total gross fixed assets	23,303,207	22,601,752
Less: Accumulated depreciation and amortization	(9,273,968)	(7,088,733)
Property and equipment, net	$ 14,029,239	$ 15,513,019

Intangible Assets - Schedule of Intangible Assets (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Intangible Assets Net Excluding Goodwill [Abstract]
Intangible assets subject to amortization, beginning balance			$ 20,464,439
Intangible assets subject to amortization, Amortization	$ (425,107)	$ (425,107)	(1,275,322)	$ (1,275,322)
Intangible assets subject to amortization, ending balance	$ 19,189,117		$ 19,189,117

Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2018	Sep. 30, 2017
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	145,000,000	145,000,000
Common stock, shares issued	87,854,651	74,785,426
Common stock, shares outstanding	87,854,651	74,785,426

Investments - Summary of Short-term and Long-term Investments (Detail) - USD ($)	Jun. 30, 2018	Sep. 30, 2017
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	$ 50,157,366	$ 40,769,539
Gross Unrealized Losses	(335,334)	(334,755)
Fair Value	49,822,032	40,434,784
Commercial Notes Due Within One Year
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	32,484,708	40,769,539
Gross Unrealized Losses	(237,007)	(334,755)
Fair Value	32,247,701	$ 40,434,784
Commercial Notes Due After One year Through Two Years
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	17,672,658
Gross Unrealized Losses	(98,327)
Fair Value	$ 17,574,331

Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Jan. 22, 2018	Jun. 30, 2018	Sep. 30, 2017
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
Preferred stock, par value		$ 0.001
Common stock, shares outstanding		87,854,651	74,785,426
Public offering price per share		$ 5.25
Underwritten Public Offering
Class Of Stock [Line Items]
Common stock shares sold	11,500,000
Public offering price per share	$ 5.25
Net proceeds, after deducting underwriting commissions and discounts and offering expenses payable	$ 56.6
2004 Equity Incentive Plan, 2013 Equity Incentive Plan, and Inducement Grants
Class Of Stock [Line Items]
Common Stock, Share reserve for issuance		9,302,968

Commitments and Contingencies - Future Minimum Lease Payments Under Operating Leases (Detail)	Jun. 30, 2018 USD ($)
Leases [Abstract]
2018 (remainder of)	$ 384,250
2019	1,435,409
2020	1,044,431
2021	1,070,496
2022	1,097,168
2023 and thereafter	4,669,328
Total	$ 9,701,082

Commitments and Contingencies - Future Principal Payments Under Note Payable (Detail)	Jun. 30, 2018 USD ($)
Long Term Notes Payable [Abstract]
2018 (remainder of)	$ 53,520
2019	223,820
2020	240,258
2021	257,903
2022	276,845
2023 and thereafter	1,326,192
Total	$ 2,378,538

Stock-Based Compensation - Summary of RSUs Activity (Detail) - Restricted Stock Units (RSUs)	9 Months Ended
	Jun. 30, 2018 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of RSUs, Unvested, beginning of period \| shares	3,108,000
Number of RSUs, Granted \| shares	1,245,500
Number of RSUs, Vested \| shares	(1,005,333)
Number of RSUs, Forfeited \| shares	(50,000)
Number of RSUs, Unvested, End of period \| shares	3,298,167
Weighted-Average Grant Date Fair Value, beginning balance \| $ / shares	$ 2.45
Weighted-Average Grant Date Fair Value, Granted \| $ / shares	3.68
Weighted-Average Grant Date Fair Value, Vested \| $ / shares	2.35
Weighted-Average Grant Date Fair Value, Forfeited \| $ / shares	1.55
Weighted-Average Grant Date Fair Value, ending balance \| $ / shares	$ 2.96

Fair Value Measurements - Fair Value Measurements for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Jun. 30, 2018	Sep. 30, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	$ 49,822,032	$ 40,434,784
Derivative liabilities		695,114
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	27,995,386	24,838,567
Short-term investments	32,247,701	40,434,784
Long-term investments	17,574,331
Derivative liabilities		695,114
Fair Value, Measurements, Recurring \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	27,995,386	24,838,567
Short-term investments	32,247,701	40,434,784
Long-term investments	$ 17,574,331
Fair Value, Measurements, Recurring \| Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liabilities		$ 695,114

Subsequent Events - Additional Information (Detail) $ in Millions	Aug. 01, 2018 USD ($)
Subsequent Event \| AMG-890 (ARO-LPA) Agreement \| Amgen
Subsequent Event [Line Items]
Amgen milestone payment	$ 10