REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of revenues
Revenues	$ 2,566,701	$ 2,419,967	$ 1,579,230
Coal sales
Disaggregation of revenues
Revenues	2,210,210	2,102,229	1,386,923
Oil & gas royalties
Disaggregation of revenues
Revenues	137,751	151,060	84,183
Transportation
Disaggregation of revenues
Revenues	142,290	113,860	69,607
Other revenues
Disaggregation of revenues
Revenues	76,450	52,818	38,517
Coal Royalties
Disaggregation of revenues
Revenues	65,614	60,680	51,471
Operating segments
Disaggregation of revenues
Revenues	2,566,701
Operating segments \| Illinois Basin Coal
Disaggregation of revenues
Revenues	1,481,556	1,296,305	919,597
Operating segments \| Illinois Basin Coal \| Coal sales
Disaggregation of revenues
Revenues	1,364,901	1,219,943	873,930
Operating segments \| Illinois Basin Coal \| Transportation
Disaggregation of revenues
Revenues	106,150	69,540	41,001
Operating segments \| Illinois Basin Coal \| Other revenues
Disaggregation of revenues
Revenues	10,505	6,822	4,666
Operating segments \| Appalachia Coal
Disaggregation of revenues
Revenues	883,334	928,087	545,539
Operating segments \| Appalachia Coal \| Coal sales
Disaggregation of revenues
Revenues	845,309	882,286	512,993
Operating segments \| Appalachia Coal \| Transportation
Disaggregation of revenues
Revenues	36,140	44,320	28,606
Operating segments \| Appalachia Coal \| Other revenues
Disaggregation of revenues
Revenues	1,885	1,481	3,940
Operating segments \| Oil & Gas Royalties
Disaggregation of revenues
Revenues	141,525	154,897	86,439
Operating segments \| Oil & Gas Royalties \| Oil & gas royalties
Disaggregation of revenues
Revenues	137,751	151,060	84,183
Operating segments \| Oil & Gas Royalties \| Other revenues
Disaggregation of revenues
Revenues	3,774	3,837	2,256
Operating segments \| Coal Royalties
Disaggregation of revenues
Revenues	42	56	69
Operating segments \| Coal Royalties \| Coal royalties
Disaggregation of revenues
Revenues	65,572	60,624	51,402
Operating segments \| Coal Royalties \| Other revenues
Disaggregation of revenues
Revenues	42	56	69
Other, Corporate and Elimination
Disaggregation of revenues
Revenues	(5,328)	(20,002)	(23,816)
Other, Corporate and Elimination \| Coal royalties
Disaggregation of revenues
Revenues	(65,572)	(60,624)	(51,402)
Other, Corporate and Elimination \| Other revenues
Disaggregation of revenues
Revenues	60,244	40,622	27,586
Elimination
Disaggregation of revenues
Revenues	(65,572)	(60,624)	(51,402)
Elimination \| Coal Royalties
Disaggregation of revenues
Revenues	$ 65,572	$ 60,624	$ 51,402