Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Income Statement [Abstract]
Revenue	$ 3,551	$ 240,735	$ 243,231
Operating expenses:
Research and development	505,870	353,188	297,307
General and administrative	98,761	92,549	124,431
Total operating expenses	604,631	445,737	421,738
Operating loss	(601,080)	(205,002)	(178,507)
Other income (expense):
Interest income	22,720	15,299	5,033
Interest expense	(32,352)	(18,326)	0
Other, net	(1,748)	1,538	765
Total other (expense) income	(11,380)	(1,489)	5,798
Loss before income tax expense and noncontrolling interest	(612,460)	(206,491)	(172,709)
Income tax (benefit) expense	(2,767)	2,784	3,785
Net loss including noncontrolling interest	(609,693)	(209,275)	(176,494)
Net loss attributable to noncontrolling interest, net of tax	(10,200)	(4,000)	(431)
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (599,493)	$ (205,275)	$ (176,063)
Net loss per share attributable to Arrowhead Pharmaceuticals, Inc.:
Basic (in dollars per share)	$ (5.00)	$ (1.92)	$ (1.67)
Diluted (in dollars per share)	$ (5.00)	$ (1.92)	$ (1.67)
Weighted-average shares used in calculating
Basic (in shares)	119,784	106,750	105,426
Diluted (in shares)	119,784	106,750	105,426
Other comprehensive loss, net of tax:
Unrealized gains (losses) on available-for-sale securities	$ 3,775	$ (2,964)	$ 0
Foreign currency translation adjustments	4,197	(122)	(67)
Comprehensive loss	$ (601,721)	$ (212,361)	$ (176,561)