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Asset Retirement Obligations (ARO Activity) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance at beginning of period $ 52,613 $ 46,002
Liabilities incurred during the period   1,920
Liabilities settled during the period (7,075) (1,900)
Accretion expense 4,040 3,528
Revision of estimated cash flows (51) 3,063
Balance at ending of period 49,527 52,613
Operating Segments | Coal Mining    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance at beginning of period 33,044 28,460
Liabilities incurred during the period   1,920
Liabilities settled during the period (6,115) (852)
Accretion expense 2,530 2,170
Revision of estimated cash flows 79 1,346
Balance at ending of period 29,538 33,044
Unallocated Items    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance at beginning of period 19,569 17,542
Liabilities incurred during the period   0
Liabilities settled during the period (960) (1,048)
Accretion expense 1,510 1,358
Revision of estimated cash flows (130) 1,717
Balance at ending of period $ 19,989 $ 19,569