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Sale of Additional Common Stock (Details) - USD ($) |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 19, 2016 | Mar. 31, 2018 | Dec. 31, 2015 | Mar. 31, 2015 | Dec. 31, 2018 | Dec. 31, 2016
Proceeds from sale of common stock |  |  |  |  | $ 245,504,000 | $ 119,269,000
Common Stock |  |  |  |  |  |
Sale of common stock |  | 8,395,000 | 5,272,750 | 8,625,000 | 8,395,000 | 5,272,750
Proceeds from sale of common stock |  | $ 245,500,000 | $ 119,300,000 | $ 115,200,000 | $ 84,000 | $ 53,000
Piper Jaffray | Common Stock | Maximum |  |  |  |  |  |
Net proceeds | $ 40,000,000 |  |  |  |  |

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