Collaboration and License Agreements - Janssen Pharmaceuticals, Inc - Additional Information (Detail)		12 Months Ended
	Oct. 03, 2018 USD ($) Target Obligation	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Oct. 31, 2021 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock			$ 250,479,000	$ 60,522,000
Revenues		$ 138,287,000	87,992,000	168,796,000
Deferred revenue		111,055,000	19,291,000
Collaboration and License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Initial transaction price	$ 252,700,000
Janssen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development, regulatory and sales milestones payments		5,300,000,000
Milestone payment	$ 70,000,000.0	20,000,000.0
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 | J N J3989 A R O H B V Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment	$ 25,000,000.0
Number of distinct performance obligations | Obligation	1
Revenues		20,200,000	65,000,000.0	167,500,000
Contract assets		100,000
Deferred revenue		0
Janssen | JNJ-75220795 (ARO-JNJ1)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues		500,000	$ 2,900,000	$ 1,000,000.0
Contract assets		10,000,000.0
Deferred revenue		0
Milestone payments		$ 10,000,000.0
Janssen | JNJ-75220795 (ARO-JNJ1) | Subsequent Event
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments					$ 10,000,000.0
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 | Collaboration and License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash received as due under collaboration agreement	$ 175,000,000.0