Net Loss Per Share - Basic and Diluted Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Numerator:
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (1,631)	$ (599,493)	$ (205,275)
Denominator:
Weighted-average basic shares outstanding (in shares)	133,758	119,784	106,750
Effect of dilutive securities (in shares)	0	0	0
Weighted-average diluted shares outstanding (in shares)	133,758	119,784	106,750
Basic net loss per share (in dollars per share)	$ (0.01)	$ (5.00)	$ (1.92)
Diluted net loss per share (in dollars per share)	$ (0.01)	$ (5.00)	$ (1.92)