Segment Information - Schedule of operating segment results (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) item	Jun. 30, 2023 USD ($)
Segment Reporting Information [Line Items]
Number of distinct lines of business | item			2
Revenues	$ 608,751	$ 757,294	$ 1,288,941	$ 1,627,225
Adjusted EBITDA	59,959	130,386	162,822	407,727
Depreciation, depletion and amortization	38,439	36,077	77,259	71,556
Accretion on asset retirement obligation	5,870	5,293	11,739	10,585
Capital expenditures	46,920	46,006	92,366	76,606
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	375,958	451,752	793,023	987,923
Adjusted EBITDA	87,276	132,839	216,811	395,896
Depreciation, depletion and amortization	31,089	28,228	62,479	56,082
Accretion on asset retirement obligation	647	615	1,294	1,229
Capital expenditures	34,132	35,639	71,166	60,459
Operating Segments | Thermal
Segment Reporting Information [Line Items]
Revenues	232,793	305,542	495,918	639,302
Adjusted EBITDA	1,067	29,179	1,999	75,434
Depreciation, depletion and amortization	7,099	7,648	14,267	15,056
Accretion on asset retirement obligation	4,810	4,314	9,620	8,628
Capital expenditures	10,953	10,042	18,497	15,535
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Adjusted EBITDA	(28,384)	(31,632)	(55,988)	(63,603)
Depreciation, depletion and amortization	251	201	513	418
Accretion on asset retirement obligation	413	364	825	728
Capital expenditures	$ 1,835	$ 325	$ 2,703	$ 612