Segment Information - Schedule of operating segment results (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($) item	Jun. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Segment Reporting Information [Line Items]
Number of distinct lines of business | item			2
Revenues	$ 757,294	$ 1,133,358	$ 1,627,225	$ 2,001,294
Adjusted EBITDA	130,386	459,967	407,727	780,950
Depreciation, depletion and amortization	36,077	32,780	71,556	64,990
Accretion on asset retirement obligation	5,293	4,430	10,585	8,860
Total assets	2,386,901	2,313,647	2,386,901	2,313,647	$ 2,433,108
Capital expenditures	46,006	30,869	76,606	53,157
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	451,752	724,492	987,923	1,196,664
Adjusted EBITDA	132,839	396,426	395,896	655,429
Depreciation, depletion and amortization	28,228	27,432	56,082	54,384
Accretion on asset retirement obligation	615	553	1,229	1,106
Total assets	1,074,408	1,019,408	1,074,408	1,019,408
Capital expenditures	35,639	25,927	60,459	43,508
Operating Segments | Thermal
Segment Reporting Information [Line Items]
Revenues	305,542	408,866	639,302	804,630
Adjusted EBITDA	29,179	93,336	75,434	193,836
Depreciation, depletion and amortization	7,648	5,134	15,056	10,166
Accretion on asset retirement obligation	4,314	3,444	8,628	6,888
Total assets	408,516	287,039	408,516	287,039
Capital expenditures	10,042	4,594	15,535	8,596
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Adjusted EBITDA	(31,632)	(29,795)	(63,603)	(68,315)
Depreciation, depletion and amortization	201	214	418	440
Accretion on asset retirement obligation	364	433	728	866
Total assets	903,977	1,007,200	903,977	1,007,200
Capital expenditures	$ 325	$ 348	$ 612	$ 1,053