Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Income Statement [Abstract]
Revenue	$ 2,500	$ 3,551
Operating expenses:
Research and development	137,002	116,491
General and administrative	26,910	23,605
Total operating expenses	163,912	140,096
Operating loss	(161,412)	(136,545)
Other income (expense):
Interest income	7,602	2,802
Interest expense	(21,646)	(5,367)
Other, net	341	421
Total other expense	(13,703)	(2,144)
Loss before income tax expense and noncontrolling interest	(175,115)	(138,689)
Income tax expense (benefit)	103	(3,313)
Net loss including noncontrolling interest	(175,218)	(135,376)
Net loss attributable to noncontrolling interest, net of tax	(2,133)	(2,512)
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (173,085)	$ (132,864)
Net loss per share attributable to Arrowhead Pharmaceuticals, Inc.:
Basic (in dollars per share)	$ (1.39)	$ (1.24)
Diluted (in dollars per share)	$ (1.39)	$ (1.24)
Weighted-average shares used in calculating
Basic (in shares)	124,848	107,415
Diluted (in shares)	124,848	107,415
Other comprehensive loss, net of tax:
Unrealized (losses) gains on available-for-sale securities	$ (507)	$ 1,909
Foreign currency translation adjustments	(106)	58
Comprehensive loss	$ (175,831)	$ (133,409)