v3.8.0.1
Document and Entity Information - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2017
Dec. 06, 2017
Mar. 31, 2017
Document And Entity Information [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Sep. 30, 2017    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Trading Symbol ARWR    
Entity Registrant Name ARROWHEAD PHARMACEUTICALS, INC.    
Entity Central Index Key 0000879407    
Current Fiscal Year End Date --09-30    
Entity Well-known Seasoned Issuer No    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Filer Category Accelerated Filer    
Entity Common Stock, Shares Outstanding   74,828,280  
Entity Public Float     $ 137