REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Disaggregation of revenues
Revenues	$ 651,697	$ 662,922
Coal sales
Disaggregation of revenues
Revenues	561,879	578,784
Oil & gas royalties
Disaggregation of revenues
Revenues	37,030	34,497
Transportation
Disaggregation of revenues
Revenues	30,753	30,238
Other revenues
Disaggregation of revenues
Revenues	22,035	19,403
Coal Royalties
Disaggregation of revenues
Revenues	18,708	15,513
Operating segments | Illinois Basin Coal
Disaggregation of revenues
Revenues	397,841	360,406
Operating segments | Illinois Basin Coal | Coal sales
Disaggregation of revenues
Revenues	370,630	336,910
Operating segments | Illinois Basin Coal | Transportation
Disaggregation of revenues
Revenues	24,476	21,328
Operating segments | Illinois Basin Coal | Other revenues
Disaggregation of revenues
Revenues	2,735	2,168
Operating segments | Appalachia Coal
Disaggregation of revenues
Revenues	198,013	251,259
Operating segments | Appalachia Coal | Coal sales
Disaggregation of revenues
Revenues	191,249	241,874
Operating segments | Appalachia Coal | Transportation
Disaggregation of revenues
Revenues	6,277	8,910
Operating segments | Appalachia Coal | Other revenues
Disaggregation of revenues
Revenues	487	475
Operating segments | Oil & Gas Royalties
Disaggregation of revenues
Revenues	37,345	35,537
Operating segments | Oil & Gas Royalties | Oil & gas royalties
Disaggregation of revenues
Revenues	37,030	34,497
Operating segments | Oil & Gas Royalties | Other revenues
Disaggregation of revenues
Revenues	315	1,040
Operating segments | Coal Royalties
Disaggregation of revenues
Revenues	6
Operating segments | Coal Royalties | Coal royalties
Disaggregation of revenues
Revenues	18,702	15,513
Operating segments | Coal Royalties | Other revenues
Disaggregation of revenues
Revenues	6
Other, Corporate and Elimination
Disaggregation of revenues
Revenues	(210)	207
Other, Corporate and Elimination | Coal royalties
Disaggregation of revenues
Revenues	(18,702)	(15,513)
Other, Corporate and Elimination | Other revenues
Disaggregation of revenues
Revenues	18,492	15,720
Elimination
Disaggregation of revenues
Revenues	(18,702)	(15,513)
Elimination | Coal Royalties
Disaggregation of revenues
Revenues	$ 18,702	$ 15,513