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Stockholders' Equity - Additional Information (Detail) - USD ($) | 6 Months Ended |
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | May 05, 2016
Stockholders Equity Note [Line Items] |  |  |
Net proceeds from issuance of common stock | $ 9,644,000 | $ 14,547,000 |
Cantor | Sales Agreement | Amendment No. 2 |  |  |
Stockholders Equity Note [Line Items] |  |  |
Maximum common stock offering price | 132,200,000 |  |
Common stock registered for sale | $ 51,400,000 |  | $ 70,000,000
Percentage of proceeds payable as compensation to underwriter | 2.00% |  |
Common stock, number of shares issued | 4,200,000 |  |
Net proceeds from issuance of common stock | $ 10,700,000 |  |

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