Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Income Statement [Abstract]
Revenue	$ 0	$ 15,825	$ 3,551	$ 224,638
Operating expenses:
Research and development	152,431	94,757	370,044	253,333
General and administrative	23,710	23,771	72,384	67,977
Total operating expenses	176,141	118,528	442,428	321,310
Operating loss	(176,141)	(102,703)	(438,877)	(96,672)
Other income (expense):
Interest income	6,498	4,172	15,550	11,414
Interest expense	(5,094)	(5,158)	(17,705)	(13,064)
Other, net	760	306	1,370	821
Total other income (expense)	2,164	(680)	(785)	(829)
Loss before income tax expense (benefit) and noncontrolling interest	(173,977)	(103,383)	(439,662)	(97,501)
Income tax expense (benefit)	0	742	(3,313)	759
Net loss including noncontrolling interest	(173,977)	(104,125)	(436,349)	(98,260)
Net loss attributable to noncontrolling interest, net of tax	(3,184)	(1,179)	(7,392)	(2,664)
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (170,793)	$ (102,946)	$ (428,957)	$ (95,596)
Net loss per share attributable to Arrowhead Pharmaceuticals, Inc.:
Basic (in dollars per share)	$ (1.38)	$ (0.96)	$ (3.63)	$ (0.90)
Diluted (in dollars per share)	$ (1.38)	$ (0.96)	$ (3.63)	$ (0.90)
Weighted-average shares used in calculating
Basic (in shares)	124,199	107,004	118,260	106,597
Diluted (in shares)	124,199	107,004	118,260	106,597
Other comprehensive loss, net of tax:
Change in unrealized losses on available-for-sale securities	$ 249	$ 0	$ 2,374	$ 0
Foreign currency translation adjustments	(141)	(79)	(139)	(275)
Comprehensive loss	$ (173,869)	$ (104,204)	$ (434,114)	$ (98,535)