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REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation of revenues (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disaggregation of revenues      
Revenues $ 1,328,129 $ 1,961,720 $ 2,002,857
Coal sales      
Disaggregation of revenues      
Revenues 1,232,272 1,762,442 1,844,808
Oil & gas royalties      
Disaggregation of revenues      
Revenues 42,912 51,735  
Transportation revenues      
Disaggregation of revenues      
Revenues 21,129 99,503 112,385
Other revenues      
Disaggregation of revenues      
Revenues 31,816 48,040 45,664
Illinois Basin      
Disaggregation of revenues      
Revenues 770,051 1,236,308 1,321,089
Illinois Basin | Coal sales      
Disaggregation of revenues      
Revenues 755,208 1,128,588 1,197,143
Illinois Basin | Transportation revenues      
Disaggregation of revenues      
Revenues 12,817 94,686 106,947
Illinois Basin | Other revenues      
Disaggregation of revenues      
Revenues 2,026 13,034 16,999
Appalachia      
Disaggregation of revenues      
Revenues 500,330 644,389 643,965
Appalachia | Coal sales      
Disaggregation of revenues      
Revenues 477,064 628,406 635,530
Appalachia | Transportation revenues      
Disaggregation of revenues      
Revenues 8,312 4,817 5,435
Appalachia | Other revenues      
Disaggregation of revenues      
Revenues 14,954 11,166 3,000
Minerals      
Disaggregation of revenues      
Revenues 43,141 53,036  
Minerals | Oil & gas royalties      
Disaggregation of revenues      
Revenues 42,912 51,735  
Minerals | Other revenues      
Disaggregation of revenues      
Revenues 229 1,301  
Other and Corporate      
Disaggregation of revenues      
Revenues 25,124 56,850 81,492
Other and Corporate | Coal sales      
Disaggregation of revenues      
Revenues   22,138 43,393
Other and Corporate | Transportation revenues      
Disaggregation of revenues      
Revenues     3
Other and Corporate | Other revenues      
Disaggregation of revenues      
Revenues 25,124 34,712 38,096
Elimination      
Disaggregation of revenues      
Revenues (10,517) (28,863) (43,689)
Elimination | Coal sales      
Disaggregation of revenues      
Revenues   (16,690) (31,258)
Elimination | Other revenues      
Disaggregation of revenues      
Revenues (10,517) (12,173) (12,431)
Elimination | Illinois Basin      
Disaggregation of revenues      
Revenues   16,690 31,191
Elimination | Appalachia      
Disaggregation of revenues      
Revenues     67
Elimination | Other and Corporate      
Disaggregation of revenues      
Revenues $ 10,517 $ 12,173 $ 12,431