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Asset Retirement Obligations (Rollforward) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]      
Balance at beginning of period (including current portion) $ 257,764 $ 252,798  
Accretion expense 21,748 19,887 $ 20,548
Obligations of divested operations   (15,455)  
Adjustments to the liability from changes in estimates (26,012) 14,889  
Reclamation work completed (39,047) (14,355)  
Balance at period end 214,453 257,764 $ 252,798
Current portion included in accrued expenses (21,781) (27,032)  
Noncurrent liability $ 192,672 $ 230,732