Net Income (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		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Numerator:
Net income (loss) attributable to Arrowhead Pharmaceuticals, Inc.	$ 370,445	$ (125,300)	$ 197,360	$ (258,164)
Denominator:
Weighted-average basic shares outstanding (in shares)	133,363	123,285	129,059	115,307
Effect of dilutive securities (in shares)	1,121	0	1,206	0
Weighted-average diluted shares outstanding (in shares)	134,484	123,285	130,265	115,307
Basic net income (loss) per share (in dollars per share)	$ 2.78	$ (1.02)	$ 1.53	$ (2.24)
Diluted net income (loss) per share (in dollars per share)	$ 2.75	$ (1.02)	$ 1.52	$ (2.24)