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Cash and Cash Equivalents (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Cash and Cash Equivalents      
Cash $ 1,103 $ 5,332  
Investments with original maturity of less than three months at the time of purchase 22,807 11,188  
Total cash and cash equivalents $ 23,910 $ 16,520 $ 20,440