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