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Summary of Significant Accounting Policies - Cash, Cash Equivalents and Restricted Cash (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Summary of Significant Accounting Policies        
Cash and cash equivalents $ 17,030 $ 16,369 $ 31,795  
Restricted cash 113 113 113  
Total $ 17,143 $ 16,482 $ 31,908 $ 23,709