SEGMENT INFORMATION - Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Reportable segment results
Revenues	$ 651,697	$ 662,922
Segment Adjusted EBITDA Expense	358,327	339,296
Segment Adjusted EBITDA	260,554	291,947
Total assets	2,965,787	2,787,749	$ 2,788,426
Capital expenditures	123,846	95,474
Other revenues
Reportable segment results
Revenues	22,035	19,403
Coal Royalties
Reportable segment results
Revenues	18,708	15,513
Operating segments | Illinois Basin Coal
Reportable segment results
Revenues	397,841	360,406
Segment Adjusted EBITDA Expense	233,087	207,069
Segment Adjusted EBITDA	140,278	132,008
Total assets	1,019,209	840,792
Capital expenditures	96,133	61,982
Operating segments | Illinois Basin Coal | Other revenues
Reportable segment results
Revenues	2,735	2,168
Operating segments | Appalachia Coal
Reportable segment results
Revenues	198,013	251,259
Segment Adjusted EBITDA Expense	117,502	125,799
Segment Adjusted EBITDA	74,235	116,550
Total assets	520,093	459,834
Capital expenditures	26,451	32,505
Operating segments | Appalachia Coal | Other revenues
Reportable segment results
Revenues	487	475
Operating segments | Oil & Gas Royalties
Reportable segment results
Revenues	37,345	35,537
Segment Adjusted EBITDA Expense	4,940	4,424
Segment Adjusted EBITDA	31,402	30,045
Total assets	783,309	768,728
Capital expenditures		2
Operating segments | Oil & Gas Royalties | Other revenues
Reportable segment results
Revenues	315	1,040
Operating segments | Coal Royalties
Reportable segment results
Revenues	6
Segment Adjusted EBITDA Expense	6,264	5,388
Segment Adjusted EBITDA	12,444	10,125
Total assets	320,217	326,064
Capital expenditures		400
Operating segments | Coal Royalties | Other revenues
Reportable segment results
Revenues	6
Other, Corporate and Elimination
Reportable segment results
Revenues	(210)	207
Segment Adjusted EBITDA Expense	(3,466)	(3,384)
Segment Adjusted EBITDA	2,195	3,219
Total assets	322,959	392,331
Capital expenditures	1,262	585
Other, Corporate and Elimination | Other revenues
Reportable segment results
Revenues	18,492	15,720
Elimination
Reportable segment results
Revenues	(18,702)	(15,513)
Elimination | Coal Royalties
Reportable segment results
Revenues	$ 18,702	$ 15,513