Revenue from Contracts with Customers - Disaggregation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Mar. 31, 2026	Dec. 31, 2025	Mar. 31, 2025	Dec. 31, 2024
Disaggregation of revenues
Revenues	$ 551,560	$ 547,463	$ 1,067,577	$ 1,087,931
Trade receivables	200,779	177,659	200,779	177,659	$ 166,573	$ 129,686	$ 177,467	$ 166,829
Coal sales
Disaggregation of revenues
Revenues	469,544	485,469	912,826	953,980
Oil & gas royalties
Disaggregation of revenues
Revenues	46,293	35,473	87,634	71,557
Coal royalties
Disaggregation of revenues
Revenues	0	0	0	0
Transportation revenues
Disaggregation of revenues
Revenues	7,510	8,558	16,153	18,758
Other revenues
Disaggregation of revenues
Revenues	28,213	17,963	50,964	43,636
Operating Segments
Disaggregation of revenues
Revenues	560,619	549,343	1,080,859	1,084,542
Operating Segments \| Illinois Basin Coal
Disaggregation of revenues
Revenues	339,472	350,247	658,434	693,242
Operating Segments \| Illinois Basin Coal \| Coal sales
Disaggregation of revenues
Revenues	330,272	343,841	640,027	677,075
Operating Segments \| Illinois Basin Coal \| Oil & gas royalties
Disaggregation of revenues
Revenues	0	0	0	0
Operating Segments \| Illinois Basin Coal \| Coal royalties
Disaggregation of revenues
Revenues	0	0	0	0
Operating Segments \| Illinois Basin Coal \| Transportation revenues
Disaggregation of revenues
Revenues	6,083	4,829	12,260	11,692
Operating Segments \| Illinois Basin Coal \| Other revenues
Disaggregation of revenues
Revenues	3,117	1,577	6,147	4,475
Operating Segments \| Appalachia Coal
Disaggregation of revenues
Revenues	151,897	145,983	292,000	285,479
Operating Segments \| Appalachia Coal \| Coal sales
Disaggregation of revenues
Revenues	139,272	141,628	272,799	276,905
Operating Segments \| Appalachia Coal \| Oil & gas royalties
Disaggregation of revenues
Revenues	0	0	0	0
Operating Segments \| Appalachia Coal \| Coal royalties
Disaggregation of revenues
Revenues	0	0	0	0
Operating Segments \| Appalachia Coal \| Transportation revenues
Disaggregation of revenues
Revenues	1,427	3,729	3,893	7,066
Operating Segments \| Appalachia Coal \| Other revenues
Disaggregation of revenues
Revenues	11,198	626	15,308	1,508
Operating Segments \| Oil & Gas Royalties
Disaggregation of revenues
Revenues	46,527	35,501	88,311	72,414
Operating Segments \| Oil & Gas Royalties \| Coal sales
Disaggregation of revenues
Revenues	0	0	0	0
Operating Segments \| Oil & Gas Royalties \| Oil & gas royalties
Disaggregation of revenues
Revenues	46,293	35,473	87,634	71,557
Operating Segments \| Oil & Gas Royalties \| Coal royalties
Disaggregation of revenues
Revenues	0	0	0	0
Operating Segments \| Oil & Gas Royalties \| Transportation revenues
Disaggregation of revenues
Revenues	0	0	0	0
Operating Segments \| Oil & Gas Royalties \| Other revenues
Disaggregation of revenues
Revenues	234	28	677	857
Operating Segments \| Coal Royalties
Disaggregation of revenues
Revenues	22,723	17,612	42,114	33,407
Operating Segments \| Coal Royalties \| Coal sales
Disaggregation of revenues
Revenues	0	0	0	0
Operating Segments \| Coal Royalties \| Oil & gas royalties
Disaggregation of revenues
Revenues	0	0	0	0
Operating Segments \| Coal Royalties \| Coal royalties
Disaggregation of revenues
Revenues	22,723	17,612	41,823	33,407
Operating Segments \| Coal Royalties \| Transportation revenues
Disaggregation of revenues
Revenues	0	0	0	0
Operating Segments \| Coal Royalties \| Other revenues
Disaggregation of revenues
Revenues	0	0	291	0
Other, Corporate and Elimination
Disaggregation of revenues
Revenues	(9,059)	(1,880)	(13,282)	3,389
Other, Corporate and Elimination \| Coal sales
Disaggregation of revenues
Revenues	0	0	0	0
Other, Corporate and Elimination \| Oil & gas royalties
Disaggregation of revenues
Revenues	0	0	0	0
Other, Corporate and Elimination \| Coal royalties
Disaggregation of revenues
Revenues	(22,723)	(17,612)	(41,823)	(33,407)
Other, Corporate and Elimination \| Transportation revenues
Disaggregation of revenues
Revenues	0	0	0	0
Other, Corporate and Elimination \| Other revenues
Disaggregation of revenues
Revenues	$ 13,664	$ 15,732	$ 28,541	$ 36,796