REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of revenues
Revenues	$ 613,569	$ 636,521	$ 1,858,616	$ 1,941,279
Coal sales
Disaggregation of revenues
Revenues	532,647	549,123	1,607,185	1,688,238
Oil & gas royalties
Disaggregation of revenues
Revenues	34,448	34,125	107,907	101,709
Transportation
Disaggregation of revenues
Revenues	24,617	34,964	82,071	95,729
Other revenues
Disaggregation of revenues
Revenues	21,857	18,309	61,453	55,603
Coal Royalties
Disaggregation of revenues
Revenues	16,647	16,763	51,943	48,843
Coal Royalties \| Coal royalties
Disaggregation of revenues
Revenues	16,647
Operating segments \| Illinois Basin Coal
Disaggregation of revenues
Revenues	356,911	371,633	1,109,513	1,087,431
Operating segments \| Illinois Basin Coal \| Coal sales
Disaggregation of revenues
Revenues	337,816	342,744	1,040,419	1,011,467
Operating segments \| Illinois Basin Coal \| Transportation
Disaggregation of revenues
Revenues	16,764	25,992	61,810	68,498
Operating segments \| Illinois Basin Coal \| Other revenues
Disaggregation of revenues
Revenues	2,331	2,897	7,284	7,466
Operating segments \| Appalachia Coal
Disaggregation of revenues
Revenues	203,164	215,800	589,390	705,301
Operating segments \| Appalachia Coal \| Coal sales
Disaggregation of revenues
Revenues	194,831	206,379	566,766	676,771
Operating segments \| Appalachia Coal \| Transportation
Disaggregation of revenues
Revenues	7,853	8,972	20,261	27,231
Operating segments \| Appalachia Coal \| Other revenues
Disaggregation of revenues
Revenues	480	449	2,363	1,299
Operating segments \| Oil & Gas Royalties
Disaggregation of revenues
Revenues	34,688	36,311	108,470	105,260
Operating segments \| Oil & Gas Royalties \| Oil & gas royalties
Disaggregation of revenues
Revenues	34,448	34,125	107,907	101,709
Operating segments \| Oil & Gas Royalties \| Other revenues
Disaggregation of revenues
Revenues	240	2,186	563	3,551
Operating segments \| Coal Royalties
Disaggregation of revenues
Revenues			10
Operating segments \| Coal Royalties \| Coal royalties
Disaggregation of revenues
Revenues		16,763	51,933	48,843
Operating segments \| Coal Royalties \| Other revenues
Disaggregation of revenues
Revenues			10
Other, Corporate and Elimination
Disaggregation of revenues
Revenues	2,159	(3,986)	(700)	(5,556)
Other, Corporate and Elimination \| Coal royalties
Disaggregation of revenues
Revenues	(16,647)	(16,763)	(51,933)	(48,843)
Other, Corporate and Elimination \| Other revenues
Disaggregation of revenues
Revenues	18,806	12,777	51,233	43,287
Elimination
Disaggregation of revenues
Revenues	(16,647)	(16,763)	(51,933)	(48,843)
Elimination \| Coal Royalties
Disaggregation of revenues
Revenues	$ 16,647	$ 16,763	$ 51,933	$ 48,843