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Organization and Significant Accounting Policies - Additional Information (Detail) |  |  |  | 1 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Oct. 07, 2020 USD ($) | Oct. 03, 2018 USD ($) Target | Sep. 28, 2016 USD ($) | Dec. 31, 2019 USD ($) | Sep. 30, 2020 USD ($) shares | Sep. 30, 2019 USD ($) shares | Dec. 31, 2018 shares | Sep. 30, 2018 USD ($) | Jul. 31, 2020 USD ($)
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Proceeds from the issuance of common stock |  |  |  | $ 250,500,000 | $ 250,478,974 | $ 60,521,739 |  | $ 56,585,035 |
Cash and cash equivalents |  |  |  |  | 143,582,667 | 221,804,128 |  |  |
Restricted Cash |  |  |  |  | 1,800,000 | 1,000,000.0 |  |  |
Short term investments |  |  |  |  | 86,889,751 | 36,899,894 |  |  |
Marketable securities |  |  |  |  | 85,000,000.0 |  |  |  |
Long term investments |  |  |  |  | 137,486,883 | 44,175,993 |  |  |
Net increase (decrease) in cash and investments |  |  |  |  | 150,100,000 |  |  |  |
Purchases of investments and marketable securities |  |  |  |  | 278,964,290 | 90,266,001 |  | 52,083,131 |
Cash outflow primarily related to operations |  |  |  |  | 95,391,570 | (173,034,923) |  | $ 47,223,417 |
Maturities of investments |  |  |  |  | 50,100,000 |  |  |  |
Fair value of contingent consideration obligation due to discontinuation of clinical trials |  |  |  |  | $ 0 | $ 0 |  |  |
Calculation of effect of dilutive stock options and restricted stock units | shares |  |  |  |  | 0 | 4,748,958 | 0 |  |
Right-of-use assets |  |  |  |  | $ 16,137,085 |  |  |  |
Right-of-use liabilities |  |  |  |  | $ 21,137,955 |  |  |  |
ASU No. 2014-09 |  |  |  |  |  |  |  |  |
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, 2018 |  |  |  |
Change in accounting principle, accounting standards update, immaterial effect [true false] |  |  |  |  | true |  |  |  |
ASU No. 2016-02 |  |  |  |  |  |  |  |  |
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, 2019 |  |  |  |
Change in accounting principle, accounting standards update, immaterial effect [true false] |  |  |  |  | true |  |  |  |
ASU No. 2018-18 |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Change in accounting principle, accounting standards update, immaterial effect [true false] |  |  |  |  | true |  |  |  |
Change in accounting principle, accounting standards update, early adoption [true false] |  |  |  |  | true |  |  |  |
Stock Options |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Anti-dilutive excluded for calculation of effect of dilutive stock options and restricted stock units | shares |  |  |  |  |  | 1,007,500 |  |  |
Restricted Stock Units |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Anti-dilutive excluded for calculation of effect of dilutive stock options and restricted stock units | shares |  |  |  |  |  | 11,500 |  |  |
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 |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Amount insured in FDIC per account |  |  |  |  | $ 250,000 |  |  |  |
Property and equipment, useful life |  |  |  |  | 7 years |  |  |  |
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 |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Property and equipment, useful life |  |  |  |  | 3 years |  |  |  |
Minimum | Takeda License Agreement |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Tiered royalties received percentage | 20.00% |  |  |  |  |  |  |  |

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