Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Income Statement [Abstract]
Revenue	$ 829,448	$ 3,551	$ 240,735
Operating expenses:
Research and development	607,159	505,870	353,188
General and administrative	123,943	98,761	92,549
Total operating expenses	731,102	604,631	445,737
Operating income (loss)	98,346	(601,080)	(205,002)
Other (expense) income:
Interest income	37,289	22,720	15,299
Interest expense	(89,361)	(32,352)	(18,326)
Other, net	5,259	(1,748)	1,538
Total other expense	(46,813)	(11,380)	(1,489)
Income (loss) before income tax expense and noncontrolling interest	51,533	(612,460)	(206,491)
Income tax expense (benefit)	21,419	(2,767)	2,784
Net income (loss) including noncontrolling interest	30,114	(609,693)	(209,275)
Net income (loss) attributable to noncontrolling interest, net of tax	31,745	(10,200)	(4,000)
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (1,631)	$ (599,493)	$ (205,275)
Net loss per share attributable to Arrowhead Pharmaceuticals, Inc.:
Basic (in dollars per share)	$ (0.01)	$ (5.00)	$ (1.92)
Diluted (in dollars per share)	$ (0.01)	$ (5.00)	$ (1.92)
Weighted-average shares used in calculating
Basic (in shares)	133,758	119,784	106,750
Diluted (in shares)	133,758	119,784	106,750
Other comprehensive loss, net of tax:
Unrealized gains (losses) on available-for-sale securities	$ 2,125	$ 3,775	$ (2,964)
Foreign currency translation adjustments	(432)	4,197	(122)
Comprehensive income (loss) attributed to noncontrolling interest	31,745	(10,200)	(4,000)
Other comprehensive income (loss)	$ 31,807	$ (601,721)	$ (212,361)