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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Restricted Cash (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Restricted Cash        
Cash and cash equivalents $ 174,479 $ 133,400 $ 121,074  
Restricted cash - current 7,234 8,242 19,500  
Restricted cash - noncurrent 23,233 27,426 315  
Cash, cash equivalents and restricted cash $ 204,946 $ 169,068 $ 140,889 $ 113,109