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Restricted Cash and Cash Equivalents (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Cash and Cash Equivalents [Abstract]        
Cash and cash equivalents $ 66,007 $ 81,377 $ 51,526  
Restricted cash and cash equivalents - current 54,931   12,452  
Restricted cash and investments - non-current 28,939   31,032  
Less Restricted investments - non-current (23,176)   (19,472)  
Total cash, cash equivalents and restricted cash and cash equivalents shown in the statement of cash flows $ 126,701 $ 133,545 $ 75,538 $ 90,357