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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Restricted Cash (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Restricted Cash        
Cash and cash equivalents $ 269,996 $ 174,479 $ 147,153 $ 133,400
Restricted cash - current   7,234 8,335 8,242
Restricted cash - noncurrent 315 23,233 25,507 27,426
Cash, cash equivalents and restricted cash $ 270,311 $ 204,946 $ 180,995 $ 169,068