Collaboration and License Agreements - Amgen, Inc - Additional Information (Detail)		3 Months Ended								12 Months Ended
	Sep. 28, 2016 USD ($) Agreement $ / shares shares	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2019 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Stock issued, price per share | $ / shares		$ 23.00				$ 5.25				$ 23.00	$ 5.25
Proceeds from the issuance of common stock										$ 60,521,739	$ 56,585,035	$ 12,419,119
Revenues		$ 43,292,770	$ 42,696,636	$ 48,148,275	$ 34,657,896	$ 11,255,000	$ 727,375	$ 650,125	$ 3,509,821	168,795,577	16,142,321	$ 31,407,709
Contract liabilities		$ 77,769,629				$ 600				$ 77,769,629	$ 600
Type of Revenue [Extensible List]		arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember
Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Stock issued, price per share | $ / shares												$ 7.16
Collaboration and License agreements | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash received as due under collaboration agreement	$ 35,000,000
Proceeds from the issuance of common stock	21,500,000
Collaboration and License agreements | Amgen | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option payments, and development, regulatory and sales milestone payments	$ 617,000,000
Development regulatory and sales milestones payments		$ 420,000,000								$ 420,000,000
Collaboration and License agreements | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Number of agreements | Agreement	2
Collaboration and License agreements | Amgen | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option payments, and development, regulatory and sales milestone payments	$ 617,000,000
Development regulatory and sales milestones payments	$ 420,000,000
AMG 890 (ARO-LPA) and ARO-AMG1 Agreement | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues										300,000	$ 16,100,000	$ 31,300,000
Contract assets		0								0
Contract liabilities		$ 0								$ 0
Type of Revenue [Extensible List]										us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Common Stock Purchase Agreement | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Shares issued | shares	3,002,793
Stock issued, price per share | $ / shares	$ 7.16