Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Income Statement [Abstract]
Revenue	$ 542,709	$ 0	$ 545,209	$ 3,551
Operating expenses:
Research and development	133,102	101,122	270,104	217,613
General and administrative	28,405	25,069	55,315	48,674
Total operating expenses	161,507	126,191	325,419	266,287
Operating income (loss)	381,202	(126,191)	219,790	(262,736)
Other income (expense):
Interest income	9,615	6,250	17,217	9,052
Interest expense	(21,639)	(7,244)	(43,285)	(12,611)
Other, net	438	189	779	610
Total other expense	(11,586)	(805)	(25,289)	(2,949)
Income (loss) before income tax expense and noncontrolling interest	369,616	(126,996)	194,501	(265,685)
Income tax expense (benefit)	1,753	0	1,856	(3,313)
Net income (loss) including noncontrolling interest	367,863	(126,996)	192,645	(262,372)
Net loss attributable to noncontrolling interest, net of tax	(2,582)	(1,696)	(4,715)	(4,208)
Net income (loss) attributable to Arrowhead Pharmaceuticals, Inc.	$ 370,445	$ (125,300)	$ 197,360	$ (258,164)
Net income (loss) per share attributable to Arrowhead Pharmaceuticals, Inc.:
Basic (in dollars per share)	$ 2.78	$ (1.02)	$ 1.53	$ (2.24)
Diluted (in dollars per share)	$ 2.75	$ (1.02)	$ 1.52	$ (2.24)
Weighted-average shares used in calculating
Basic (in shares)	133,363	123,285	129,059	115,307
Diluted (in shares)	134,484	123,285	130,265	115,307
Other comprehensive income (loss), net of tax:
Unrealized gains on available-for-sale securities	$ 653	$ 216	$ 146	$ 2,125
Foreign currency translation adjustments	(400)	(56)	(506)	2
Comprehensive income (loss)	$ 368,116	$ (126,836)	$ 192,285	$ (260,245)