Collaboration and License Agreements - Amgen, Inc - Additional Information (Detail)		1 Months Ended		3 Months Ended								12 Months Ended
	Sep. 28, 2016 USD ($) Agreement $ / shares shares	Nov. 30, 2016 USD ($)	Sep. 30, 2016 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2016 USD ($) $ / shares	Aug. 01, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Stock issued, price per share | $ / shares				$ 5.25								$ 5.25
Proceeds from the issuance of common stock												$ 56,585,035	$ 12,419,119	$ 52,231,433
Revenues				$ 11,255,000	$ 727,375	$ 650,125	$ 3,509,821	$ 8,713,785	$ 9,342,498	$ 8,985,930	$ 4,365,496	$ 16,142,321	$ 31,407,709	$ 158,333
Type of Revenue [Extensible List]												arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember
Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Stock issued, price per share | $ / shares			$ 7.16					$ 7.16					$ 7.16	$ 7.16
Collaboration and License agreements | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Number of agreements | Agreement	2
ARO-AMG1 Agreement | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional amount of common stock shares agreed to sell upon exercise of option	$ 5,000,000
Number of trading days, used to calculate share price of Common Stock, surrounding option exercise date	30 days
Cash received as due under collaboration agreement			$ 5,000,000
Deferred revenue			$ 5,000,000											$ 5,000,000
Deferred revenue recognized into revenue												$ 2,500,000	$ 2,500,000
Collaboration and License agreements | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option payments, and development, regulatory and sales milestone payments.	$ 617,000,000
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
AMG-890 (ARO-LPA) Agreement | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash received as due under collaboration agreement		$ 30,000,000
Deferred revenue		$ 30,000,000
Deferred revenue recognized into revenue												2,700,000	27,300,000
Amgen milestone payment															$ 10,000,000
AMG 890 (ARO-LPA) and ARO-AMG1 Agreement | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues												$ 1,000,000	$ 1,500,000	$ 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