Quarterly report pursuant to Section 13 or 15(d)

Basis of Presentation (Details)

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Basis of Presentation (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Current Assets:        
Cash and cash equivalents $ 134,032 $ 60,115 $ 27,730  
Restricted cash included in prepaid expenses and other current assets 228 2,000 2,000  
Total cash, cash equivalents and restricted cash $ 134,260 $ 62,115 $ 29,730 $ 54,592