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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Schedule of Restricted Cash and Cash Equivalents (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2021
Dec. 31, 2020
Restricted Cash And Cash Equivalents [Roll Forward]    
Restricted cash, beginning of period $ 20,817 $ 20,081
Additional collateral posted 23 768
Refunds of collateral 0 (32)
Restricted cash, end of period $ 20,840 $ 20,817