REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Disaggregation of revenues
Revenues	$ 2,406,511	$ 1,569,976	$ 1,328,129
Illinois Basin
Disaggregation of revenues
Revenues	1,296,305	919,597	769,957
Appalachia
Disaggregation of revenues
Revenues	928,087	545,539	500,330
Oil & Gas Royalties
Disaggregation of revenues
Revenues	141,441	77,185	43,141
Coal Royalties
Disaggregation of revenues
Revenues	60,680	51,471	42,217
Other, Corporate and Elimination
Disaggregation of revenues
Revenues	(20,002)	(23,816)	(27,516)
Coal sales
Disaggregation of revenues
Revenues	2,102,229	1,386,923	1,232,272
Coal sales | Illinois Basin
Disaggregation of revenues
Revenues	1,219,943	873,930	755,208
Coal sales | Appalachia
Disaggregation of revenues
Revenues	882,286	512,993	477,064
Oil & gas royalties
Disaggregation of revenues
Revenues	138,402	74,988	42,912
Oil & gas royalties | Oil & Gas Royalties
Disaggregation of revenues
Revenues	138,402	74,988	42,912
Coal royalties | Coal Royalties
Disaggregation of revenues
Revenues	60,624	51,402	42,112
Coal royalties | Other, Corporate and Elimination
Disaggregation of revenues
Revenues	(60,624)	(51,402)	(42,112)
Transportation
Disaggregation of revenues
Revenues	113,860	69,607	21,129
Transportation | Illinois Basin
Disaggregation of revenues
Revenues	69,540	41,001	12,817
Transportation | Appalachia
Disaggregation of revenues
Revenues	44,320	28,606	8,312
Other revenues
Disaggregation of revenues
Revenues	52,020	38,458	31,816
Other revenues | Illinois Basin
Disaggregation of revenues
Revenues	6,822	4,666	1,932
Other revenues | Appalachia
Disaggregation of revenues
Revenues	1,481	3,940	14,954
Other revenues | Oil & Gas Royalties
Disaggregation of revenues
Revenues	3,039	2,197	229
Other revenues | Coal Royalties
Disaggregation of revenues
Revenues	56	69	105
Other revenues | Other, Corporate and Elimination
Disaggregation of revenues
Revenues	$ 40,622	$ 27,586	$ 14,596