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RELATED-PARTY TRANSACTIONS - Affiliate Royalty Agreements (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Related Party Transaction      
As of the beginning of period $ 53,901    
As of the end of period 60,959 $ 53,901  
Coal lease      
Related Party Transaction      
As of the beginning of period 41,216 32,645 $ 32,993
Payments 13,819 15,317 10,818
Recoupment (4,078) (4,178) (3,271)
Unrecoupable (2,568) (2,568) (7,895)
As of the end of period 48,389 41,216 32,645
SGP | Tunnel Ridge      
Related Party Transaction      
As of the beginning of period 1,500   3,000
Payments 3,000 4,500  
Recoupment (3,000) (3,000) (3,000)
As of the end of period 1,500 1,500  
WKY CoalPlay | December 2014 coal lease - Henderson and Union Counties, Kentucky      
Related Party Transaction      
As of the beginning of period 16,603 14,077 10,684
Payments 3,597 3,597 3,597
Recoupment (1,022) (1,071) (204)
As of the end of period 19,178 16,603 14,077
WKY CoalPlay | December 2014 coal lease - Webster County, Kentucky      
Related Party Transaction      
As of the beginning of period     5,356
Payments 2,568 2,568 2,570
Recoupment     (31)
Unrecoupable (2,568) (2,568) (7,895)
WKY CoalPlay | December 2014 coal lease - Henderson County, Kentucky      
Related Party Transaction      
As of the beginning of period 12,607 10,086 7,566
Payments 2,522 2,521 2,520
As of the end of period 15,129 12,607 10,086
WKY CoalPlay | February 2015 coal lease - Henderson and Union Counties, Kentucky      
Related Party Transaction      
As of the beginning of period 10,506 8,482 6,387
Payments 2,132 2,131 2,131
Recoupment (56) (107) (36)
As of the end of period $ 12,582 $ 10,506 $ 8,482