Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Numerator:
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (132,864)	$ (41,325)
Denominator:
Weighted-average basic shares outstanding (in shares)	107,415	106,039
Effect of dilutive securities (in shares)	0	0
Weighted-average diluted shares outstanding (in shares)	107,415	106,039
Basic (in dollars per share)	$ (1.24)	$ (0.39)
Diluted (in dollars per share)	$ (1.24)	$ (0.39)
Potentially dilutive securities excluded from computation of diluted earnings per share (in shares)	3,544	3,327