Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Income Statement [Abstract]
Revenue	$ 264,033	$ 2,500
Operating expenses:
Research and development	177,203	137,002
General and administrative	46,021	26,910
Total operating expenses	223,224	163,912
Operating income (loss)	40,809	(161,412)
Other (expense) income:
Interest income	9,690	7,602
Interest expense	(22,505)	(21,646)
Other, net	277	341
Total other expense	(12,538)	(13,703)
Income (loss) before income tax expense and noncontrolling interest	28,271	(175,115)
Income tax expense	29	103
Net income (loss) including noncontrolling interest	28,242	(175,218)
Net loss attributable to noncontrolling interest, net of tax	(2,569)	(2,133)
Net income (loss) attributable to Arrowhead Pharmaceuticals, Inc.	$ 30,811	$ (173,085)
Net income (loss) per share attributable to Arrowhead Pharmaceuticals, Inc.:
Basic (in dollars per share)	$ 0.22	$ (1.39)
Diluted (in dollars per share)	$ 0.22	$ (1.39)
Weighted-average shares used in calculating
Basic (in shares)	137,039	124,848
Diluted (in shares)	140,706	124,848
Other comprehensive income (loss), net of tax:
Unrealized gains (losses) on available-for-sale securities	$ 146	$ (507)
Foreign currency translation adjustments	110	(106)
Comprehensive loss attributed to noncontrolling interest	(2,569)	(2,133)
Other comprehensive income (loss)	$ 28,498	$ (175,831)