Consolidated Statements of Operations and Comprehensive (Loss) Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Income Statement [Abstract]
Revenue	$ 0	$ 146,267	$ 3,551	$ 208,813
Operating expenses:
Research and development	101,122	74,881	217,613	158,576
General and administrative	25,069	23,221	48,674	44,206
Total operating expenses	126,191	98,102	266,287	202,782
Operating (loss) income	(126,191)	48,165	(262,736)	6,031
Other income (expense):
Interest income	6,250	4,560	9,052	7,242
Interest expense	(7,244)	(5,057)	(12,611)	(7,906)
Other, net	189	8	610	515
Total other expense	(805)	(489)	(2,949)	(149)
(Loss) income before income tax (benefit) expense and noncontrolling interest	(126,996)	47,676	(265,685)	5,882
Income tax (benefit) expense	0	0	(3,313)	17
Net (loss) income including noncontrolling interest	(126,996)	47,676	(262,372)	5,865
Net loss attributable to noncontrolling interest, net of tax	(1,696)	(999)	(4,208)	(1,485)
Net (loss) income attributable to Arrowhead Pharmaceuticals, Inc.	$ (125,300)	$ 48,675	$ (258,164)	$ 7,350
Net (loss) income per share attributable to Arrowhead Pharmaceuticals, Inc.:
Basic (in dollars per share)	$ (1.02)	$ 0.46	$ (2.24)	$ 0.07
Diluted (in dollars per share)	$ (1.02)	$ 0.45	$ (2.24)	$ 0.07
Weighted-average shares used in calculating
Basic (in shares)	123,285	106,757	115,307	106,394
Diluted (in shares)	123,285	108,143	115,307	107,893
Other comprehensive (loss) income, net of tax:
Change in unrealized losses on available-for-sale securities	$ 216	$ 0	$ 2,125	$ 0
Foreign currency translation adjustments	(56)	(74)	2	(196)
Comprehensive (loss) income	$ (126,836)	$ 47,602	$ (260,245)	$ 5,669