Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Income Statement [Abstract]
Revenue	$ 146,267	$ 151,805	$ 208,813	$ 179,244
Operating expenses:
Research and development	74,881	75,985	158,576	141,750
General and administrative	23,221	34,267	44,206	59,262
Total operating expenses	98,102	110,252	202,782	201,012
Operating income (loss)	48,165	41,553	6,031	(21,768)
Other income (expense):
Interest income	4,560	1,054	7,242	2,210
Interest expense	(5,057)	0	(7,906)	0
Other, net	8	1,759	515	1,052
Total other (loss) income	(489)	2,813	(149)	3,262
Income (loss) before income tax expense and noncontrolling interest	47,676	44,366	5,882	(18,506)
Income tax expense	0	0	17	0
Net income (loss) including noncontrolling interest	47,676	44,366	5,865	(18,506)
Net loss attributable to noncontrolling interest, net of tax	(999)	0	(1,485)	0
Net income (loss) attributable to Arrowhead Pharmaceuticals, Inc.	$ 48,675	$ 44,366	$ 7,350	$ (18,506)
Net income (loss) per share attributable to Arrowhead Pharmaceuticals, Inc.:
Basic (in dollars per share)	$ 0.46	$ 0.42	$ 0.07	$ (0.18)
Diluted (in dollars per share)	$ 0.45	$ 0.41	$ 0.07	$ (0.18)
Weighted-average shares used in calculating
Basic (in shares)	106,757	105,545	106,394	105,034
Diluted (in shares)	108,143	107,929	107,893	105,034
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments	$ (74)	$ 1	$ (196)	$ (38)
Comprehensive income (loss)	$ 47,602	$ 44,367	$ 5,669	$ (18,544)