Collaboration and License Agreements - Amgen, Inc - Additional Information (Detail)		12 Months Ended
	Sep. 28, 2016 USD ($) Agreement	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jul. 31, 2020 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
Type of Revenue [Extensible List]		us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Contract liabilities		$ 111,055,000	$ 19,291,000
Collaboration and License agreements | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Number of agreements | Agreement	2
Cash received as due under collaboration agreement	$ 35,000,000.0
Olpasiran Agreement | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments	30,000,000.0
Olpasiran Agreement | Amgen | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional remaining development regulatory and sales milestones payments		400,000,000.0
License Collaboration And Stock Purchase Agreement | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock	$ 21,500,000
Olpasiran and ARO-AMG1 Agreement | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments					$ 20,000,000
Revenues		$ 0	$ 20,100,000	$ 300,000
Type of Revenue [Extensible List]		us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Contract liabilities		$ 0
Contract assets		$ 0