Net Loss Per Share - 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, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Numerator:
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (170,793)	$ (102,946)	$ (428,957)	$ (95,596)
Denominator:
Weighted-average basic shares outstanding (in shares)	124,199	107,004	118,260	106,597
Effect of dilutive securities (in shares)	0	0	0	0
Weighted-average diluted shares outstanding (in shares)	124,199	107,004	118,260	106,597
Basic net loss per share (in dollars per share)	$ (1.38)	$ (0.96)	$ (3.63)	$ (0.90)
Diluted net loss per share (in dollars per share)	$ (1.38)	$ (0.96)	$ (3.63)	$ (0.90)