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Supplemental Financial Information - Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]          
Asset retirement obligation, beginning of period     $ 21,447 $ 2,721 $ 2,721
Asset retirement obligations assumed     0   21,328
Accretion     901   543
Liabilities settled     (7,522)   (3,565)
Changes due to amount and timing of reclamation     (1,942)   420
Asset retirement obligations, end of period $ 12,884   12,884   21,447
Less current portion 4,856   4,856   9,370
Asset retirement obligations, long-term 8,028   8,028   $ 12,077
Gain on change in estimate, asset retirement obligation $ 0 $ 0 $ 1,942 $ 0