Collaboration and License Agreements - Janssen Pharmaceuticals, Inc - Additional Information (Detail)		3 Months Ended		9 Months Ended
	Oct. 03, 2018 USD ($) Target Obligation	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	May 31, 2021 USD ($)	Sep. 30, 2020 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock					$ 250,479,000
Revenues		$ 45,891,000	$ 27,376,000	$ 100,004,000	80,359,000
Contract liabilities, current deferred revenue		150,934,000		150,934,000			$ 19,291,000
Collaboration and License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Initial transaction price	$ 252,700,000
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
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment	$ 60,000,000.0
Milestone payments						$ 10,000,000
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
Number of distinct performance obligations | Obligation	1
Revenues		0	6,900,000	20,200,000	57,900,000
Contract assets		400,000		400,000
Contract liabilities, current deferred revenue		0		0
Janssen | ARO-JNJ1
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment						$ 10,000,000
Revenues		200,000	$ 400,000	500,000	$ 2,400,000
Contract assets		300,000		300,000
Contract liabilities, current deferred revenue		$ 0		$ 0