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Subsequent Events (Details) - Common Stock [Member] - USD ($) $ / shares in Units, $ in Thousands | 2 Months Ended | 12 Months Ended
--- | --- | ---
Feb. 20, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Stock Transactions [Abstract] |  |  |  |
Number of shares of common stock sold (in shares) |  | 7,510,555 | 730,444 | 4,760,594
ATM Agreement [Member] |  |  |  |
Stock Transactions [Abstract] |  |  |  |
Number of shares of common stock sold (in shares) |  | 7,510,555 | 730,444 | 4,760,594
Average sales price per common share (in dollars per share) |  | $ 3.71 | $ 5.19 | $ 4.16
Aggregate proceeds from sales of common stock |  | $ 27,837 | $ 3,790 | $ 19,791
Sales commissions, fees and other costs associated with issuance of common stock |  | $ 785 | $ 113 | $ 594
Subsequent Event [Member] | ATM Agreement [Member] |  |  |  |
Stock Transactions [Abstract] |  |  |  |
Number of shares of common stock sold (in shares) | 560,338 |  |  |
Average sales price per common share (in dollars per share) | $ 5.24 |  |  |
Aggregate proceeds from sales of common stock | $ 2,936 |  |  |
Sales commissions, fees and other costs associated with issuance of common stock | $ 88 |  |  |

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