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Organization and Significant Accounting Policies - Additional Information (Detail) |  | 1 Months Ended | 3 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 28, 2016 USD ($) $ / shares shares | Oct. 31, 2018 USD ($) Target $ / shares shares | Dec. 31, 2018 USD ($) shares | Dec. 31, 2017 USD ($) shares | Sep. 30, 2018 USD ($) | Aug. 01, 2018 USD ($) | Sep. 30, 2017 USD ($)
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Proceeds from the issuance of common stock |  |  | $ 60,521,729 |  |  |  |
Cash and cash equivalents |  |  | 189,772,981 | $ 11,531,345 | $ 30,133,213 |  | $ 24,838,567
Short term investments |  |  | 53,980,307 |  | 46,400,176 |  |
Long term investments |  |  | 59,595,287 |  |  |  |
Net increase (decrease) in cash and investments |  |  | 226,800,000 |  |  |  |
Promissory note payable |  |  | 2,300,000 |  |  |  |
Restricted Cash |  |  | 0 |  | 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 |  |  | 4,498,360 | 0 |  |  |
Stock Options and Restricted Stock Units |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Anti-dilutive excluded for calculation of effect of dilutive stock options | shares |  |  | 680,000 |  |  |  |
Collaboration and License agreements | Amgen |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cash received as due under collaboration agreement | $ 35,000,000 |  |  |  |  |  |
Proceeds from the issuance of common stock | $ 21,500,000 |  |  |  |  |  |
Janssen and JJDC | Collaboration and License agreements |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cash received as due under collaboration agreement |  | $ 175,000,000 |  |  |  |  |
Proceeds from the issuance of common stock |  | $ 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 |  |  |  |  |
Amgen | AMG-890 (ARO-LPA) Agreement |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Amgen 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 | Janssen and JJDC |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Shares issued | shares |  | 3,260,869 |  |  |  |  |
Stock issued, price per share | $ / shares |  | $ 23.00 |  |  |  |  |
Proceeds from the issuance of common stock |  | $ 75,000,000 |  |  |  |  |
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, Plant and Equipment, Useful Life |  |  | 3 years |  |  |  |
Maximum |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Amount insured in FDIC per account |  |  | $ 250,000 |  |  |  |
Property, Plant and Equipment, Useful Life |  |  | 7 years |  |  |  |
Maximum | Collaboration and License agreements | Amgen |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Option payments, and development, regulatory and sales milestone payments. | $ 617,000,000 |  |  |  |  |  |
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 |  |  |  |  |

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