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Collaboration and License Agreements - Amgen, Inc - Additional Information (Detail)		3 Months Ended
	Sep. 28, 2016 USD ($) Agreement	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)	Jul. 31, 2020 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock			$ 250,477,000
Revenues		$ 21,303,000	$ 29,455,000
Type of Revenue [Extensible List]		arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantAndProductSalesMember
Contract liabilities		$ 6,744,000		$ 19,291,000
Collaboration and License agreements \| Amgen Incorporated
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Number of agreements \| Agreement	2
Cash received as due under collaboration agreement	$ 35,000,000.0
Olpasiran Agreement \| Amgen Incorporated
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments	30,000,000.0
Olpasiran Agreement \| Amgen Incorporated \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional remaining development regulatory and sales milestones payments		400,000,000.0
License Collaboration and Stock Purchase Agreement \| Amgen Incorporated
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock	$ 21,500,000
Olpasiran and ARO-AMG1 Agreement \| Amgen Incorporated
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments					$ 20,000,000.0
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Contract assets		$ 0
Contract liabilities		$ 0