Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Numerator:
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (205,275)	$ (176,063)	$ (140,848)
Denominator:
Weighted-average basic shares outstanding (in shares)	106,750	105,426	103,745
Effect of dilutive securities (in shares)	0	0	0
Weighted-average diluted shares outstanding (in shares)	106,750	105,426	103,745
Basic (in dollars per share)	$ (1.92)	$ (1.67)	$ (1.36)
Diluted (in dollars per share)	$ (1.92)	$ (1.67)	$ (1.36)
Potentially dilutive securities excluded from computation of diluted earnings per share (in shares)	4,053	3,885	2,063