Net Loss Per Share - Schedule of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Numerator:
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (173,085)	$ (132,864)
Denominator:
Weighted-average basic shares outstanding (in shares)	124,848	107,415
Effect of dilutive securities (in shares)	0	0
Weighted-average diluted shares outstanding (in shares)	124,848	107,415
Basic net loss per share (in dollars per share)	$ (1.39)	$ (1.24)
Diluted net loss per share (in dollars per share)	$ (1.39)	$ (1.24)