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Collaboration and License Agreements - Janssen Pharmaceuticals, Inc - Additional Information (Detail)		3 Months Ended
	Oct. 03, 2018 USD ($) Target Obligation	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock			$ 250,477,000
Revenues		$ 21,303,000	29,455,000
Deferred revenue		6,744,000		$ 19,291,000
Collaboration and License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Initial transaction price	$ 252,600,000
JJDC \| Common Stock Purchase Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Oct. 03, 2018
Proceeds from the issuance of common stock	$ 75,000,000.0
Janssen and JJDC \| Collaboration and License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash received as due under collaboration agreement	$ 175,000,000.0
Janssen \| License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Oct. 03, 2018
Janssen \| License Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development, regulatory and sales milestones payments	$ 1,600,000,000
Janssen \| Research Collaboration and Option Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Oct. 03, 2018
Number of additional targets for development, regulatory and sales milestone payments \| Target	3
Janssen \| Collaboration Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development, regulatory and sales milestones payments	$ 1,900,000,000
Janssen \| JNJ-3989 (ARO-HBV) Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment	$ 25,000,000.0	25,000,000.0	25,000,000.0
Number of distinct performance obligations \| Obligation	1
Revenues		12,700,000	28,800,000
Contract assets		0
Deferred revenue		6,700,000
Janssen \| ARO-JNJ1
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues		300,000	$ 700,000
Contract assets		800,000
Deferred revenue		$ 0