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Stockholders' Equity - Additional Information (Detail) - USD ($) |  | 1 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jan. 08, 2018 | Jan. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | May 05, 2016
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Common stock, shares issued and sold |  |  | 136,853,000 | 115,555,000 |  |
Common stock, par value |  |  | $ 0.001 | $ 0.001 |  |
Proceeds from common stock sold |  |  | $ 85,036,000 | $ 30,197,000 | $ 22,121,000 |
Preferred stock, shares issued |  |  | 0 | 0 |  |
Stockholders rights plan expiration date |  |  | Oct. 27, 2019 |  |  |
Stockholder Rights Plan |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Minimum percentage of common stock acquired by stockholders |  |  | 15.00% |  |  |
Designated preferred stock for future issuance |  |  | 250,000 |  |  |
Series C Junior Participating preferred stock |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Preferred stock, shares issued |  |  | 0 |  |  |
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 |  |  | 27,900,000 | $ 30,300,000 |  |
Maximum common stock offering price | $ 70,000,000 |  |  |  |  |
Common stock registered for sale |  |  | $ 41,600,000 |  |  |
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 |  |  | 4,200,000 | 11,000,000 |  |
Common Stock |  |  |  |  |  |
Stockholders Equity Note [Line Items] |  |  |  |  |  |
Common stock, number of shares issued |  |  | 18,202,000 | 10,986,000 | 3,526,000 |
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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