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Cash and Cash Equivalents (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Apr. 01, 2017
Dec. 31, 2016
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents $ 42,628 $ 39,775 $ 26,318 $ 50,359
Cash        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents 38,840 35,999    
Money market funds        
Cash and Cash Equivalents [Line Items]        
Cash and cash equivalents $ 3,788 $ 3,776