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Organization and Significant Accounting Policies - Additional Information (Detail)			1 Months Ended	6 Months Ended
	Oct. 03, 2018 USD ($) Target	Sep. 28, 2016 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock			$ 250,500,000	$ 250,478,974	$ 60,521,729
Cash and cash equivalents				256,650,727		$ 221,804,128
Restricted Cash				1,800,000
Short term investments				50,959,058		36,899,894
Long term investments				190,618,075		$ 44,175,993
Net increase (decrease) in cash and investments				195,400,000
Right-of-use assets				10,263,197
Right-of-use liabilities				14,867,189
Janssen | JNJ-3989 (ARO-HBV) Agreement
Organization And Significant Accounting Policies [Line Items]
Milestone payment	$ 25,000,000			25,000,000
Janssen | Collaboration Agreement
Organization And Significant Accounting Policies [Line Items]
Number of additional targets for development and sales milestone payments | Target	3
Janssen and JJDC | Collaboration and License agreements
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement	$ 175,000,000
JJDC | Common Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock	75,000,000
Amgen | License Collaboration and Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement		$ 35,000,000
Proceeds from the issuance of common stock		21,500,000
Amgen | AMG-890 (ARO-LPA) Agreement
Organization And Significant Accounting Policies [Line Items]
Milestone payment		$ 10,000,000
Maximum | Janssen | License Agreement
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments	1,600,000,000
Maximum | Janssen | Collaboration Agreement
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments	$ 1,900,000,000
Maximum | Amgen | AMG-890 (ARO-LPA) Agreement
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments				$ 420,000,000

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