Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Summary of Components of Marketable Securities (Details)

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Summary of Significant Accounting Policies - Summary of Components of Marketable Securities (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Amortized cost $ 18,687  
Unrealized gain  
Unrealized loss 78  
Market value 18,765
U.S. Government Obligations [Member]    
Amortized cost 1,744  
Unrealized gain  
Unrealized loss  
Market value 1,744  
Corporate Bonds [Member]    
Amortized cost 16,943  
Unrealized gain  
Unrealized loss 78  
Market value $ 17,021