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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Schedule of Restricted Cash and Cash Equivalents (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Restricted Cash And Cash Equivalents [Roll Forward]    
Restricted cash, beginning of period $ 22,127 $ 23,175
Refunds of collateral (2,592) (14,657)
Additional collateral posted 117 13,609
Restricted cash, end of period $ 19,652 $ 22,127