REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation of revenues
Revenues	$ 593,350	$ 641,836	$ 1,245,047	$ 1,304,758
Coal sales
Disaggregation of revenues
Revenues	512,659	560,331	1,074,538	1,139,115
Oil & gas royalties
Disaggregation of revenues
Revenues	36,429	33,087	73,459	67,584
Transportation
Disaggregation of revenues
Revenues	26,701	30,527	57,454	60,765
Other revenues
Disaggregation of revenues
Revenues	17,561	17,891	39,596	37,294
Coal Royalties
Disaggregation of revenues
Revenues	16,588	16,567	35,296	32,080
Coal Royalties \| Coal royalties
Disaggregation of revenues
Revenues	16,584
Coal Royalties \| Other revenues
Disaggregation of revenues
Revenues	4
Operating segments \| Illinois Basin Coal
Disaggregation of revenues
Revenues	354,761	355,392	752,602	715,798
Operating segments \| Illinois Basin Coal \| Coal sales
Disaggregation of revenues
Revenues	331,973	331,813	702,603	668,723
Operating segments \| Illinois Basin Coal \| Transportation
Disaggregation of revenues
Revenues	20,570	21,178	45,046	42,506
Operating segments \| Illinois Basin Coal \| Other revenues
Disaggregation of revenues
Revenues	2,218	2,401	4,953	4,569
Operating segments \| Appalachia Coal
Disaggregation of revenues
Revenues	188,213	238,242	386,226	489,501
Operating segments \| Appalachia Coal \| Coal sales
Disaggregation of revenues
Revenues	180,686	228,518	371,935	470,392
Operating segments \| Appalachia Coal \| Transportation
Disaggregation of revenues
Revenues	6,131	9,349	12,408	18,259
Operating segments \| Appalachia Coal \| Other revenues
Disaggregation of revenues
Revenues	1,396	375	1,883	850
Operating segments \| Oil & Gas Royalties
Disaggregation of revenues
Revenues	36,437	33,412	73,782	68,949
Operating segments \| Oil & Gas Royalties \| Oil & gas royalties
Disaggregation of revenues
Revenues	36,429	33,087	73,459	67,584
Operating segments \| Oil & Gas Royalties \| Other revenues
Disaggregation of revenues
Revenues	8	325	323	1,365
Operating segments \| Coal Royalties
Disaggregation of revenues
Revenues	4		10
Operating segments \| Coal Royalties \| Coal royalties
Disaggregation of revenues
Revenues		16,567	35,286	32,080
Operating segments \| Coal Royalties \| Other revenues
Disaggregation of revenues
Revenues			10
Other, Corporate and Elimination
Disaggregation of revenues
Revenues	(2,649)	(1,777)	(2,859)	(1,570)
Other, Corporate and Elimination \| Coal royalties
Disaggregation of revenues
Revenues	(16,584)	(16,567)	(35,286)	(32,080)
Other, Corporate and Elimination \| Other revenues
Disaggregation of revenues
Revenues	13,935	14,790	32,427	30,510
Elimination
Disaggregation of revenues
Revenues	(16,584)	(16,567)	(35,286)	(32,080)
Elimination \| Coal Royalties
Disaggregation of revenues
Revenues	$ 16,584	$ 16,567	$ 35,286	$ 32,080