RELATED-PARTY TRANSACTIONS - Affiliate Royalty Agreements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Related Party Transaction
As of the beginning of period	$ 55,613
As of the end of period	60,608	$ 55,613
Coal lease
Related Party Transaction
As of the beginning of period	55,613	48,389	$ 41,216
Payments	11,250	13,817	13,819
Recoupment	(6,255)	(4,025)	(4,078)
Unrecoupable		(2,568)	(2,568)
As of the end of period	60,608	55,613	48,389
Tunnel Ridge
Related Party Transaction
As of the beginning of period	1,500	1,500	1,500
Payments	3,000	3,000	3,000
Recoupment	(3,000)	(3,000)	(3,000)
As of the end of period	1,500	1,500	1,500
WKY CoalPlay | December 2014 coal lease - Henderson and Union Counties, Kentucky
Related Party Transaction
As of the beginning of period	21,750	19,178	16,603
Payments	3,597	3,597	3,597
Recoupment	(3,255)	(1,025)	(1,022)
As of the end of period	22,092	21,750	19,178
WKY CoalPlay | December 2014 coal lease - Webster County, Kentucky
Related Party Transaction
Payments		2,568	2,568
Unrecoupable		(2,568)	(2,568)
WKY CoalPlay | December 2014 coal lease - Henderson County, Kentucky
Related Party Transaction
As of the beginning of period	17,650	15,129	12,607
Payments	2,522	2,521	2,522
As of the end of period	20,172	17,650	15,129
WKY CoalPlay | February 2015 coal lease - Henderson and Union Counties, Kentucky
Related Party Transaction
As of the beginning of period	14,713	12,582	10,506
Payments	2,131	2,131	2,132
Recoupment			(56)
As of the end of period	$ 16,844	$ 14,713	$ 12,582