Segment Information - Schedule of operating segment results (Details)	12 Months Ended
	Dec. 31, 2022 USD ($) item segment	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) segment
Segment Reporting Information [Line Items]
Number of reportable segments | segment	2		3
Number of remaining active thermal mines | item	3
Number of distinct lines of business	2
Revenues	$ 3,724,593,000	$ 2,208,042,000	$ 1,467,592,000
Adjusted EBITDA	1,260,432,000	533,430,000	23,743,000
Depreciation, depletion and amortization	133,300,000	120,327,000	121,552,000
Accretion on asset retirement obligation	17,721,000	21,748,000	19,887,000
Total assets	2,433,108,000	2,117,160,000	1,722,472,000
Capital expenditures	172,728,000	245,440,000	285,821,000
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	2,157,710,000	1,149,133,000	641,536,000
Adjusted EBITDA	1,021,932,000	442,830,000	91,322,000
Depreciation, depletion and amortization	111,772,000	99,171,000	91,202,000
Accretion on asset retirement obligation	2,213,000	2,030,000	1,943,000
Total assets	1,058,217,000	964,761,000	811,605,000
Capital expenditures	140,031,000	227,802,000	269,273,000
Operating Segments | Thermal
Segment Reporting Information [Line Items]
Revenues	1,566,883,000	1,057,480,000	801,632,000
Adjusted EBITDA	353,884,000	175,709,000	34,035,000
Depreciation, depletion and amortization	20,650,000	20,231,000	28,351,000
Accretion on asset retirement obligation	13,775,000	17,675,000	15,368,000
Total assets	381,099,000	205,147,000	196,336,000
Capital expenditures	28,578,000	5,949,000	10,719,000
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	0	1,429,000	24,424,000
Adjusted EBITDA	(115,384,000)	(85,109,000)	(101,614,000)
Depreciation, depletion and amortization	878,000	925,000	1,999,000
Accretion on asset retirement obligation	1,733,000	2,043,000	2,576,000
Total assets	993,792,000	947,252,000	714,531,000
Capital expenditures	$ 4,119,000	$ 11,689,000	$ 5,829,000