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Stockholders' Equity - Additional Information (Detail) - USD ($) |  | 1 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
Jan. 08, 2018 | Jan. 31, 2018 | Sep. 30, 2018 | Sep. 30, 2017 | May 05, 2016
Stockholders Equity Note [Line Items] |  |  |  |  |
Proceeds from common stock sold |  |  | $ 79,301,000 | $ 18,840,000 |
Amendment No. 2 | Cantor | Sales Agreement |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |
Maximum common stock offering price |  |  | 132,200,000 |  |
Common stock registered for sale |  |  |  |  | $ 70,000,000
Amendment No. 3 | Cantor | Sales Agreement |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |
Proceeds from common stock sold |  |  | 22,300,000 |  |
Maximum common stock offering price | $ 70,000,000 |  |  |  |
Common stock registered for sale |  |  | $ 47,300,000 |  |
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 |  |  | 3,200,000 |  |
Common Stock | Underwritten Public Offering |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |
Common stock, shares issued and sold |  | 14,030,000 |  |  |
Common stock, par value |  | $ 0.001 |  |  |
Common stock, sale of stock, price per share |  | $ 4.10 |  |  |
Proceeds from common stock sold |  | $ 57,200,000 |  |  |

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