Document and Entity Information - USD ($)
$ in Billions
12 Months Ended
Sep. 30, 2019
Nov. 20, 2019
Mar. 31, 2019
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Sep. 30, 2019    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Trading Symbol ARWR    
Entity Central Index Key 0000879407    
Current Fiscal Year End Date --09-30    
Entity Well-known Seasoned Issuer Yes    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding   95,708,027  
Entity Public Float     $ 1.7
Entity Interactive Data Current Yes    
Title of 12(b) Security Common Stock    
Security Exchange Name NASDAQ    
Entity File Number 001-38042    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 46-0408024    
Entity Address, Address Line One 177 E. Colorado Blvd    
Entity Address, Address Line Two Suite 700    
Entity Address, City or Town Pasadena    
Entity Address, State or Province CA    
Entity Address, Postal Zip Code 91105    
City Area Code 626    
Local Phone Number 304-3400    
Document Annual Report true    
Document Transition Report false    
Documents Incorporated by Reference

Portions of the Definitive Proxy Statement to be filed for Arrowhead Pharmaceuticals Inc.’s 2020 Annual Meeting of Stockholders are incorporated by reference into Part III hereof.