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PROPERTY, PLANT AND EQUIPMENT - Summary (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost $ 3,608,347 $ 3,554,090  
Less accumulated depreciation, depletion and amortization (1,909,669) (1,753,845)  
Total property, plant and equipment, net 1,698,678 1,800,245  
Depreciation 256,900 297,000 $ 312,400
Mining Equipment      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 1,896,470 1,896,324  
Land And Mineral Rights      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 458,440 454,310  
Coal reserves not subject to depletion 37,400 37,500  
Oil and gas mineral interests      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 647,864 616,904  
Unproved properties 355,100 340,500  
Buildings, Office Equipment And Improvements      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 282,902 279,938  
Construction and mine development in progress      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost 44,217 25,799  
Mine development costs      
PROPERTY, PLANT AND EQUIPMENT      
Property, plant and equipment, at cost $ 278,454 280,815  
Mine development costs   $ 13,100