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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, 2020
Dec. 31, 2019
Restricted Cash And Cash Equivalents [Roll Forward]    
Restricted cash, beginning of period $ 20,081 $ 19,652
 Refunds of collateral (32) 0
Collateral Already Posted, Aggregate Fair Value 768 429
Restricted cash, end of period $ 20,817 $ 20,081