Subsequent Events - Additional Information (Detail) $ in Thousands	Nov. 22, 2021 USD ($)	Nov. 19, 2021 USD ($) ft² Option	Sep. 30, 2021 USD ($)
Subsequent Event [Line Items]
Estimated lease payments			$ 37,114
Subsequent Event | GSK | Collaboration and License agreements
Subsequent Event [Line Items]
Milestone payment	$ 120,000
Milestone payment receivable at start of phase two	30,000
Milestone payment receivable upon achievement of phase two and first patient dosed in phase three	$ 100,000
Tiered royalties on net product sales receive description	Arrowhead is further eligible to receive tiered royalties on net product sales in a range of mid-teens to twenty percent.
Subsequent Event | Maximum | GSK | Collaboration and License agreements
Subsequent Event [Line Items]
Commercial milestone payments at first commercial sale	$ 190,000
Sales-related milestone payments	$ 590,000
Research Facility in San Diego | California | Subsequent Event
Subsequent Event [Line Items]
Lease term		15 years
Office space leases, in square feet | ft²		144,000
Lease commencement date		2023-05
Estimated lease payments		$ 119,000
Estimated payments for operating expenses		3,000
Payments for leasehold improvements, net of tenant improvement allowances		$ 31,000
Operating lease, description		Pursuant to the lease, within twelve months of the expiration of the initial 15-year term, the Company has the option to extend the lease for up to one additional ten-year term, with certain annual increases in base rent
Operating lease renewal term		10 years
Research Facility in San Diego | California | Subsequent Event | Maximum
Subsequent Event [Line Items]
Number of options to renew | Option		1