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Summary of Significant Accounting Policies - Summary of Components of Marketable Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Amortized Cost $ 6,829 $ 18,687
Unrealized Gains
Unrealized Losses (53) (78)
Market Value 6,776 18,765
U.S. Treasuries [Member]    
Amortized Cost 929 1,744
Unrealized Gains
Unrealized Losses
Market Value 929 1,744
Corporate Bonds [Member]    
Amortized Cost 5,900 16,943
Unrealized Gains
Unrealized Losses (53) (78)
Market Value $ 5,847 $ 17,021