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Cash, Cash Equivalents and Restricted Cash (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Cash and Cash Equivalents [Abstract]        
Lease agreement term (in years) 1 year      
Balance Sheet        
Cash and cash equivalents $ 53,504 $ 46,156 $ 67,325  
Restricted cash 159,010 136,667 128,524  
Statement of Cash Flows        
Cash, cash equivalents and restricted cash $ 212,514 $ 182,823 $ 195,849 $ 250,241