Consolidated Statements of Operations and Comprehensive (Loss) Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Income Statement [Abstract]
Revenue	$ 27,767	$ 0	$ 572,976	$ 3,551
Operating expenses:
Research and development	162,368	152,431	432,472	370,044
General and administrative	30,949	23,710	86,264	72,384
Total operating expenses	193,317	176,141	518,736	442,428
Operating (loss) income	(165,550)	(176,141)	54,240	(438,877)
Other (expense) income:
Interest income	11,019	6,498	28,236	15,550
Interest expense	(24,382)	(5,094)	(67,667)	(17,705)
Other, net	(176)	760	603	1,370
Total other (expense) income	(13,539)	2,164	(38,828)	(785)
(Loss) income before income tax expense and noncontrolling interest	(179,089)	(173,977)	15,412	(439,662)
Income tax (benefit) expense	(437)	0	1,419	(3,313)
Net (loss) income including noncontrolling interest	(178,652)	(173,977)	13,993	(436,349)
Net loss attributable to noncontrolling interest, net of tax	(3,411)	(3,184)	(8,126)	(7,392)
Net (loss) income attributable to Arrowhead Pharmaceuticals, Inc.	$ (175,241)	$ (170,793)	$ 22,119	$ (428,957)
Net (loss) income per share attributable to Arrowhead Pharmaceuticals, Inc.:
Basic (in dollars per share)	$ (1.26)	$ (1.38)	$ 0.17	$ (3.63)
Diluted (in dollars per share)	$ (1.26)	$ (1.38)	$ 0.17	$ (3.63)
Weighted-average shares used in calculating
Basic (in shares)	139,039	124,199	132,385	118,260
Diluted (in shares)	139,039	124,199	133,352	118,260
Other comprehensive income (loss), net of tax:
Unrealized gains on available-for-sale securities	$ 876	$ 249	$ 1,022	$ 2,374
Foreign currency translation adjustments	91	(141)	(415)	(139)
Comprehensive (loss) income	$ (177,685)	$ (173,869)	$ 14,600	$ (434,114)