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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Schedule of Restricted Cash and Cash Equivalents (Details) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Restricted Cash And Cash Equivalents [Roll Forward]    
Restricted cash, beginning of period $ 22,127 $ 23,175
Additional collateral posted 102 13,609
Release of collateral related to change in surety agents (674) (14,657)
Restricted cash, end of period $ 21,555 $ 22,127