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Organization and Significant Accounting Policies - Additional Information (Detail) |  |  |  | 1 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Oct. 07, 2020 USD ($) | Oct. 03, 2018 USD ($) Target | Sep. 28, 2016 USD ($) | Jan. 31, 2021 USD ($) | Mar. 31, 2021 USD ($) | Mar. 31, 2020 USD ($) | Sep. 30, 2020 USD ($) | Jul. 31, 2020 USD ($)
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Proceeds from the issuance of common stock |  |  |  |  |  | $ 250,479,000 |  |
Cash and cash equivalents |  |  |  |  | $ 372,377,000 |  | $ 143,583,000 |
Restricted Cash |  |  |  |  | 2,400,000 |  |  |
Short-term investments |  |  |  |  | 79,148,000 |  | 86,890,000 |
Marketable securities |  |  |  |  | 125,800,000 |  |  |
Long-term investments |  |  |  |  | 97,490,000 |  | $ 137,487,000 |
Net increase (decrease) in cash and investments |  |  |  |  | $ 221,800,000 |  |  |
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] |  |  |  |  |  |  |  |
Cash received as due under agreement |  |  |  | $ 300,000,000.0 |  |  |  |
Profit sharing structure ratio | 50.00% |  |  |  |  |  |  |
Development regulatory and commercial milestones payments | $ 740,000,000.0 |  |  |  |  |  |  |
Janssen | JNJ-3989 (ARO-HBV) Agreement |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Milestone payment |  | $ 25,000,000.0 |  |  | $ 25,000,000.0 |  |  |
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% |  |  |  |  |  |  |

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