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PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost $ 4,435,535 $ 4,172,544  
Less accumulated depreciation, depletion and amortization (2,269,265) (2,149,881)  
Total property, plant and equipment, net 2,166,270 2,022,663  
Depreciation $ 287,100 276,400 $ 273,800
Minimum      
PROPERTY, PLANT AND EQUIPMENT      
Estimated useful life 1 year    
Maximum      
PROPERTY, PLANT AND EQUIPMENT      
Estimated useful life 33 years    
Mining equipment and processing facilities      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost $ 2,131,697 1,989,541  
Land and coal mineral rights      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 508,892 504,736  
Coal reserves not subject to depletion 14,100 13,400  
Oil & gas mineral interests      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 878,083 853,350  
Unproved properties 377,500 411,600  
Buildings, office equipment, improvements and other miscellaneous equipment      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 315,092 310,876  
Construction, mine development and other projects in progress      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 213,579 184,895  
Mine development costs      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 388,192 329,146  
Mine development costs $ 67,200 $ 44,400