Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Income Statement [Abstract]
Revenue	$ 240,735,000	$ 243,231,000	$ 138,287,000
Operating expenses:
Research and development	353,188,000	297,307,000	206,342,000
General and administrative	92,549,000	124,431,000	80,981,000
Total operating expenses	445,737,000	421,738,000	287,323,000
Operating loss	(205,002,000)	(178,507,000)	(149,036,000)
Other income (expense):
Interest income	15,299,000	5,033,000	6,120,000
Interest expense	(18,326,000)	0	0
Other, net	1,538,000	765,000	2,070,000
Total other (expense) income	(1,489,000)	5,798,000	8,190,000
Loss before income tax expense and noncontrolling interest	(206,491,000)	(172,709,000)	(140,846,000)
Income tax expense	2,784,000	3,785,000	2,000
Net loss including noncontrolling interest	(209,275,000)	(176,494,000)	(140,848,000)
Net loss attributable to noncontrolling interest, net of tax	(4,000,000)	(431,000)	0
Net loss attributable to Arrowhead Pharmaceuticals, Inc.	$ (205,275,000)	$ (176,063,000)	$ (140,848,000)
Net loss per share attributable to Arrowhead Pharmaceuticals, Inc.:
Basic (in dollars per share)	$ (1.92)	$ (1.67)	$ (1.36)
Diluted (in dollars per share)	$ (1.92)	$ (1.67)	$ (1.36)
Weighted-average shares used in calculating
Basic (in shares)	106,750	105,426	103,745
Diluted (in shares)	106,750	105,426	103,745
Other comprehensive loss, net of tax:
Unrealized losses on available-for-sale securities	$ (2,964,000)	$ 0	$ 0
Foreign currency translation adjustments	(122,000)	(67,000)	(87,000)
Comprehensive loss	$ (212,361,000)	$ (176,561,000)	$ (140,935,000)