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Summary of Significant Accounting Polices - Cash and Cash Equivalents and Fair Value of Financial Instruments (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Cash and Cash Equivalents        
Cash $ 2,781 $ 1,173    
Money market fund 13,588 22,423    
Total 16,369 23,596 $ 62,780 $ 14,668
Fair Value of Financial Instruments        
Transfers of assets or liabilities between levels $ 0 $ 0