Collaboration and License Agreements - Amgen, Inc - Additional Information (Detail)		3 Months Ended		9 Months Ended
	Sep. 28, 2016 USD ($) Agreement	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jul. 31, 2020 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock					$ 250,479,000
Revenues		$ 45,891,000	$ 27,376,000	$ 100,004,000	$ 80,359,000
Type of Revenue [Extensible List]		arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantAndProductSalesMember
Contract liabilities		$ 150,934,000		$ 150,934,000		$ 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		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
Revenues		$ 0	$ 20,000,000.0	$ 0	$ 20,000,000.0
Type of Revenue [Extensible List]		us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Contract assets		$ 0		$ 0
Contract liabilities		$ 0		$ 0