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Organization and Significant Accounting Policies - Additional Information (Detail) |  | 1 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 28, 2016 USD ($) $ / shares shares | Oct. 31, 2018 USD ($) Target $ / shares shares | Sep. 30, 2019 USD ($) $ / shares shares | Sep. 30, 2018 USD ($) $ / shares shares | Sep. 30, 2017 USD ($) $ / shares shares | Aug. 31, 2019 USD ($) | Apr. 30, 2019 USD ($) | Oct. 03, 2018 USD ($) | Aug. 01, 2018 USD ($)
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Stock issued, price per share | $ / shares |  |  | $ 23.00 | $ 5.25 |  |  |  |  |
Proceeds from the issuance of common stock |  |  | $ 60,521,739 | $ 56,585,035 | $ 12,419,119 |  |  |  |
Cash and cash equivalents |  |  | 221,804,128 | 30,133,213 |  |  |  |  |
Short term investments |  |  | 36,899,894 | 46,400,176 |  |  |  |  |
Long-term investments |  |  | 44,175,993 |  |  |  |  |  |
Net increase (decrease) in cash and investments |  |  | 226,300,000 |  |  |  |  |  |
Restricted Cash |  |  | 1,000,000 | 0 |  |  |  |  |
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 |  |  | 4,748,958 | 0 | 0 |  |  |  |
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 |  |  |  |  |  |
JJDC | Collaboration and License agreements |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Cash received as due under agreement |  | $ 175,000,000 | $ 175,000,000 |  |  |  |  |  |
Proceeds from the issuance of common stock |  | $ 75,000,000 | 75,000,000 |  |  |  |  |  |
Janssen | Collaboration Agreement |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Number of additional targets for development and sales milestone payments | Target |  | 3 |  |  |  |  |  |  |
Janssen | JNJ-3989 (ARO-HBV) Agreement |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Milestone payment |  |  | $ 50,000,000 |  |  | $ 25,000,000 | $ 25,000,000 | $ 25,000,000 |
Amgen |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Stock issued, price per share | $ / shares |  |  |  |  | $ 7.16 |  |  |  |
Amgen | License Collaboration and Stock Purchase Agreement |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Cash received as due under agreement | $ 35,000,000 |  |  |  |  |  |  |  |
Proceeds from the issuance of common stock | $ 21,500,000 |  |  |  |  |  |  |  |
Amgen | AMG-890 (ARO-LPA) Agreement |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Milestone payment |  |  |  |  |  |  |  |  | $ 10,000,000
Common Stock Purchase Agreement | JJDC |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Shares issued | shares |  | 3,260,869 |  |  |  |  |  |  |
Stock issued, price per share | $ / shares |  | $ 23.00 |  |  |  |  |  |  |
Common Stock Purchase Agreement | Amgen |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Shares issued | shares | 3,002,793 |  |  |  |  |  |  |  |
Stock issued, price per share | $ / shares | $ 7.16 |  |  |  |  |  |  |  |
Minimum |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Property and equipment, useful life |  |  | 3 years |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Amount insured in FDIC per account |  |  | $ 250,000 |  |  |  |  |  |
Property and equipment, useful life |  |  | 7 years |  |  |  |  |  |
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 | Collaboration and License agreements |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Development regulatory and sales milestones payments | $ 420,000,000 |  |  |  |  |  |  |  |
Option payments, and development, regulatory and sales milestone payments | $ 617,000,000 |  |  |  |  |  |  |  |

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