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PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost $ 4,172,544 $ 3,931,422  
Less accumulated depreciation, depletion and amortization (2,149,881) (2,050,754)  
Total property, plant and equipment, net 2,022,663 1,880,668  
Depreciation $ 276,400 273,800 $ 260,300
Minimum      
PROPERTY, PLANT AND EQUIPMENT      
Estimated useful life 1 year    
Maximum      
PROPERTY, PLANT AND EQUIPMENT      
Estimated useful life 20 years    
Mining Equipment      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost $ 1,989,541 1,927,603  
Land And Mineral Rights      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 504,736 499,950  
Coal reserves not subject to depletion 13,400 29,900  
Oil and gas mineral interests      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 853,350 814,667  
Unproved properties 411,600 422,700  
Buildings, Office Equipment And Improvements      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 310,876 300,436  
Construction and mine development in progress      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 184,895 99,042  
Mine development costs      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 329,146 289,724  
Mine development costs $ 44,400 $ 11,300