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Stockholders' Equity - Additional Information (Detail) - USD ($) shares in Millions |  | 9 Months Ended
--- | --- | ---
Jan. 08, 2018 | Sep. 30, 2019 | Sep. 30, 2018
Stockholders Equity Note [Line Items] |  |  |
Net proceeds from public offering |  | $ 17,803,000 | $ 79,301,000
Cantor Fitzgerald & Co | Amendment No. 3 | Sales Agreement |  |  |
Stockholders Equity Note [Line Items] |  |  |
Maximum common stock offering price | $ 70,000,000.0 |  |
Percentage of proceeds payable as compensation to underwriter | 2.00% |  |
Unsold shares of common stock, value | $ 31,400,000 |  |
Common stock, number of shares issued |  | 3.5 |
Net proceeds from public offering |  | $ 17,700,000 |
Common stock registered for sale |  | $ 23,500,000 |

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