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RELATED-PARTY TRANSACTIONS - Affiliate Royalty Agreements (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Related Party Transaction      
As of the beginning of period $ 44,197    
As of the end of period 53,901 $ 44,197  
Coal lease      
Related Party Transaction      
As of the beginning of period 32,645 32,993 $ 19,820
Payments 15,317 10,818 16,819
Recoupment (4,178) (3,271) (3,646)
Unrecoupable (2,568) (7,895)  
As of the end of period 41,216 32,645 32,993
SGP | Tunnel Ridge      
Related Party Transaction      
As of the beginning of period   3,000  
Payments 4,500   6,000
Recoupment (3,000) (3,000) (3,000)
As of the end of period 1,500   3,000
WKY CoalPlay | December 2014 coal lease - Henderson and Union Counties, Kentucky      
Related Party Transaction      
As of the beginning of period 14,077 10,684 7,195
Payments 3,597 3,597 3,598
Recoupment (1,071) (204) (109)
As of the end of period 16,603 14,077 10,684
WKY CoalPlay | December 2014 coal lease - Webster County, Kentucky      
Related Party Transaction      
As of the beginning of period   5,356 3,319
Payments 2,568 2,570 2,568
Recoupment   (31) (531)
Unrecoupable (2,568) (7,895)  
As of the end of period     5,356
WKY CoalPlay | December 2014 coal lease - Henderson County, Kentucky      
Related Party Transaction      
As of the beginning of period 10,086 7,566 5,044
Payments 2,521 2,520 2,522
As of the end of period 12,607 10,086 7,566
WKY CoalPlay | February 2015 coal lease - Henderson and Union Counties, Kentucky      
Related Party Transaction      
As of the beginning of period 8,482 6,387 4,262
Payments 2,131 2,131 2,131
Recoupment (107) (36) (6)
As of the end of period $ 10,506 $ 8,482 $ 6,387