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Basis of Presentation and Summary of Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]        
Cash and cash equivalents $ 43,052 $ 363,177    
Restricted cash and cash equivalents in prepaid expenses and other current assets 1,000 3,985    
Cash and cash equivalents and restricted cash and cash equivalents at end of period $ 44,052 $ 367,162 $ 1,098,171 $ 126,782