Net (Loss) Income Per Share - Schedule of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Numerator:
Net (loss) income attributable to Arrowhead Pharmaceuticals, Inc.	$ (175,241)	$ (170,793)	$ 22,119	$ (428,957)
Denominator:
Weighted-average basic shares outstanding (in shares)	139,039	124,199	132,385	118,260
Effect of dilutive securities (in shares)	0	0	967	0
Weighted-average diluted shares outstanding (in shares)	139,039	124,199	133,352	118,260
Basic net (loss) income per share (in dollars per share)	$ (1.26)	$ (1.38)	$ 0.17	$ (3.63)
Diluted net (loss) income per share (in dollars per share)	$ (1.26)	$ (1.38)	$ 0.17	$ (3.63)