Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Income Statement [Abstract]
Revenue	$ 3,551	$ 62,546
Operating expenses:
Research and development	116,491	83,695
General and administrative	23,605	20,985
Total operating expenses	140,096	104,680
Operating loss	(136,545)	(42,134)
Other income (expense):
Interest income	2,802	2,682
Interest expense	(5,367)	(2,849)
Other, net	421	507
Total other (expense) income	(2,144)	340
Loss before income tax (benefit) expense and noncontrolling interest	(138,689)	(41,794)
Income tax (benefit) expense	(3,313)	17
Net loss including noncontrolling interest	(135,376)	(41,811)
Net loss attributable to noncontrolling interest, net of tax	(2,512)	(486)
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (132,864)	$ (41,325)
Net loss per share attributable to Arrowhead Pharmaceuticals, Inc.:
Basic (in dollars per share)	$ (1.24)	$ (0.39)
Diluted (in dollars per share)	$ (1.24)	$ (0.39)
Weighted-average shares used in calculating
Basic (in shares)	107,415	106,039
Diluted (in shares)	107,415	106,039
Other comprehensive loss, net of tax:
Change in unrealized losses on available-for-sale securities	$ 1,909	$ 0
Foreign currency translation adjustments	58	(122)
Comprehensive loss	$ (133,409)	$ (41,933)