Net Income (Loss) Per Share - Schedule of Computation of Basic and Diluted Net (Loss) Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Numerator:
Net income (loss) attributable to Arrowhead Pharmaceuticals, Inc.	$ 30,811	$ (173,085)
Denominator:
Weighted-average basic shares outstanding (in shares)	137,039	124,848
Effect of dilutive securities (in shares)	3,667	0
Weighted-average diluted shares outstanding (in shares)	140,706	124,848
Basic net income (loss) per share (in dollars per share)	$ 0.22	$ (1.39)
Diluted net income (loss) per share (in dollars per share)	$ 0.22	$ (1.39)