Unaudited Quarterly Financial Data - Summary of Unaudited Quarterly Financial Data (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 7,632,856	$ 27,375,778	$ 23,528,853	$ 29,454,579	$ 43,292,770	$ 42,696,636	$ 48,148,275	$ 34,657,896	$ 87,992,066	$ 168,795,577	$ 16,142,321
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
Operating Income (Loss)	$ (50,119,092)	$ (15,945,862)	$ (22,240,255)	$ (4,853,594)	$ 9,638,049	$ 18,595,749	$ 22,010,692	$ 10,946,144	$ (93,158,803)	$ 61,190,634	$ (55,936,235)
Net Income (Loss)	$ (48,432,715)	$ (13,611,213)	$ (19,835,570)	$ (2,673,728)	$ 11,704,906	$ 20,335,708	$ 23,896,982	$ 12,037,253	$ (84,553,226)	$ 67,974,849	$ (54,450,478)
Net Income (Loss) per share-Basic	$ (0.48)	$ (0.13)	$ (0.20)	$ (0.03)	$ 0.12	$ 0.21	$ 0.25	$ 0.13	$ (0.84)	$ 0.72	$ (0.65)
Net Income (Loss) per share-Diluted	$ (0.48)	$ (0.13)	$ (0.20)	$ (0.03)	$ 0.12	$ 0.21	$ 0.24	$ 0.13	$ (0.84)	$ 0.69	$ (0.65)