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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, 2021
Dec. 31, 2020
Restricted Cash And Cash Equivalents [Roll Forward]    
Restricted cash, beginning of period $ 20,817 $ 20,081
Additional collateral posted 36 768
Refunds of collateral (560) (32)
Restricted cash, end of period $ 20,293 $ 20,817