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Stockholders' Equity - Additional Information (Detail) - USD ($) shares in Millions | 6 Months Ended |
--- | --- | ---
Jun. 30, 2016 | Jun. 30, 2015 | May 05, 2016
Stockholders Equity Note [Line Items] |  |  |
Proceeds from common stock sold | $ 14,547,000 | $ 75,324,000 |
Cantor | Sales Agreement |  |  |
Stockholders Equity Note [Line Items] |  |  |
Maximum common stock offering price | 132,200,000 |  |
Common stock registered for sale | $ 70,000,000 |  | $ 70,000,000
Percentage of proceeds payable as compensation to underwriter | 2.00% |  |
Common stock, number of shares issued | 2.4 |  |
Proceeds from common stock sold | $ 14,500,000 |  |

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