Organization and Significant Accounting Policies - Additional Information (Detail)				1 Months Ended	3 Months Ended
	Oct. 07, 2020 USD ($)	Oct. 03, 2018 USD ($) Target	Sep. 28, 2016 USD ($)	Jan. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)	Jul. 31, 2020 USD ($)
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock						$ 250,477,000
Cash and cash equivalents					$ 139,921,000		$ 143,583,000
Restricted Cash					2,400,000
Short term investments					79,394,000		86,890,000
Marketable securities					86,000,000.0
Long term investments					110,855,000		$ 137,487,000
Net increase (decrease) in cash and investments					$ (36,800,000)
ASU No. 2018-18
Organization And Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted [true false]					true
Change in accounting principle, accounting standards update, adoption date					Oct. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect [true false]					true
ASU No. 2018-15
Organization And Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted [true false]					true
Change in accounting principle, accounting standards update, adoption date					Oct. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect [true false]					true
Takeda License Agreement
Organization And Significant Accounting Policies [Line Items]
Profit sharing structure ratio	50.00%
Development regulatory and commercial milestones payments	$ 740,000,000.0
Takeda License Agreement | Subsequent Event
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement				$ 300,000,000.0
Janssen | JNJ-3989 (ARO-HBV) Agreement
Organization And Significant Accounting Policies [Line Items]
Milestone payment		$ 25,000,000.0			$ 25,000,000.0	$ 25,000,000.0
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		$ 175,000,000.0
JJDC | Common Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock		75,000,000.0
Amgen Incorporated | Olpasiran Agreement
Organization And Significant Accounting Policies [Line Items]
Milestone payment								$ 20,000,000.0
Milestone payments			$ 30,000,000.0
Amgen Incorporated | License Collaboration and Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement			35,000,000.0
Proceeds from the issuance of common stock			$ 21,500,000
Maximum | Takeda License Agreement
Organization And Significant Accounting Policies [Line Items]
Tiered royalties received percentage	25.00%
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 Incorporated | Olpasiran Agreement
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments					$ 400,000,000.0
Minimum | Takeda License Agreement
Organization And Significant Accounting Policies [Line Items]
Tiered royalties received percentage	20.00%