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Cash and Marketable Securities - Schedule of Restricted Cash and Cash Equivalents (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Cash and Cash Equivalents [Abstract]        
Cash and cash equivalents $ 39,661 $ 77,123 $ 44,001  
Restricted cash included within other non-current assets 1,500 1,500 451  
Total cash, cash equivalents, and restricted cash $ 41,161 $ 78,623 $ 44,452 $ 218,910