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Stockholders' Equity - Additional Information (Detail) - USD ($) | 1 Months Ended | 3 Months Ended |
--- | --- | --- | ---
Oct. 31, 2018 | Dec. 31, 2018 | Sep. 30, 2018
Class Of Stock [Line Items] |  |  |
Capital stock authorized for issuance |  | 150,000,000 |
Common stock, shares authorized |  | 145,000,000 | 145,000,000
Common stock, par value |  | $ 0.001 | $ 0.001
Preferred stock, shares authorized |  | 5,000,000 |
Preferred stock, par value |  | $ 0.001 |
Common stock, shares outstanding |  | 92,591,457 | 88,505,302
Proceeds from the issuance of common stock |  | $ 60,521,729 |
Janssen and JJDC | Common Stock Purchase Agreement |  |  |
Class Of Stock [Line Items] |  |  |
Shares issued | 3,260,869 |  |
Proceeds from the issuance of common stock | $ 75,000,000 |  |
Stock issued, price per share | $ 23.00 |  |
2004 Equity Incentive Plan, 2013 Equity Incentive Plan, and Inducement Grants |  |  |
Class Of Stock [Line Items] |  |  |
Common Stock, Share reserve for issuance |  | 7,954,283 |

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