Segment Information - Schedule of Segment Reporting Information by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	$ 1,002,543	$ 553,432	$ 3,122,310	$ 1,590,841
Revenues	1,002,543	553,432	3,122,310	1,590,841
Adjusted EBITDA	182,384	204,365	552,505	545,014
Other Profit or Loss	679	1,042	2,291	3,676
Depreciation, Depletion and Amortization	(151,006)	(53,329)	(441,825)	(165,173)
General and Administrative Costs	(34,133)	(36,151)	(158,285)	(77,669)
Interest Expense	(10,606)	(4,664)	(28,676)	(15,063)
Interest Income	7,578	5,016	20,297	14,145
Loss on Debt Extinguishment	0	0	(11,680)	0
Non-Service Related Pension and Postretirement Benefit Costs	(6,562)	(4,400)	(19,301)	(13,201)
Idle Mine Costs	(5,080)	(1,291)	(14,644)	(4,003)
Other Operating Expense (Income), net	(1,742)	7,685	3,169	17,608
Other Cost of Sales	(2,912)	(5,102)	(28,184)	(6,341)
(Loss) Earnings Before Income Tax	(21,400)	113,171	(124,333)	298,993
Segment Assets	6,195,888	2,824,117	6,195,888	2,824,117	$ 2,879,543
Depreciation, Depletion and Amortization	151,006	53,329	441,825	165,173
Capital Expenditures	49,297	39,388	203,304	137,148
Operating Segments
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	1,013,559	565,885	3,159,977	1,616,513
Intersegment Revenues
Segment Reporting Information [Line Items]
Revenue Not from Contract with Customer	(14,187)	(16,250)	(48,488)	(37,599)
Corporate and Other
Segment Reporting Information [Line Items]
Revenue Not from Contract with Customer	3,171	3,797	10,821	11,927
Depreciation, Depletion and Amortization	(22,590)	(7,691)	(69,673)	(26,216)
Segment Assets	1,360,773	827,573	1,360,773	827,573
Depreciation, Depletion and Amortization	22,590	7,691	69,673	26,216
Capital Expenditures	956	1,356	10,631	5,336
High CV Thermal
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	514,721	517,720	1,663,307	1,469,201
High CV Thermal | Operating Segments
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	514,721	517,720	1,663,307	1,469,201
Cash Costs of Revenue	293,002	245,252	927,621	717,948
Transportation Costs	82,550	77,990	275,363	228,842
Other Segment Items	0	(1,701)	0	(3,021)
Adjusted EBITDA	139,169	196,179	460,323	525,432
Depreciation, Depletion and Amortization	(52,842)	(42,306)	(156,153)	(129,146)
Segment Assets	2,179,909	1,778,921	2,179,909	1,778,921
Depreciation, Depletion and Amortization	52,842	42,306	156,153	129,146
Capital Expenditures	40,642	30,743	122,812	114,744
High CV Thermal | Intersegment Revenues
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	0	0	0	0
Metallurgical
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	292,694	24,425	897,268	87,024
Metallurgical | Operating Segments
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	292,694	24,425	897,268	87,024
Cash Costs of Revenue	209,285	31,394	634,425	99,862
Transportation Costs	66,912	1,666	210,982	7,645
Other Segment Items	(975)	0	56,674	0
Adjusted EBITDA	17,472	(8,635)	(4,813)	(20,483)
Depreciation, Depletion and Amortization	(65,381)	(2,056)	(186,587)	(6,267)
Segment Assets	2,140,661	130,021	2,140,661	130,021
Depreciation, Depletion and Amortization	65,381	2,056	186,587	6,267
Capital Expenditures	2,719	4,484	57,192	9,463
Metallurgical | Intersegment Revenues
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	0	0	0	0
PRB
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	186,366	0	535,827	0
PRB | Operating Segments
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	186,366	0	535,827	0
Cash Costs of Revenue	169,945	0	471,349	0
Transportation Costs	2,830	0	8,030	0
Other Segment Items	0	0	0	0
Adjusted EBITDA	13,591	0	56,448	0
Depreciation, Depletion and Amortization	(8,795)	0	(25,241)	0
Segment Assets	422,024	0	422,024	0
Depreciation, Depletion and Amortization	8,795	0	25,241	0
Capital Expenditures	1,876	0	6,785	0
PRB | Intersegment Revenues
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	0	0	0	0
Baltimore Marine Terminal
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	5,591	7,490	15,087	22,689
Baltimore Marine Terminal | Operating Segments
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	19,778	23,740	63,575	60,288
Cash Costs of Revenue	7,626	6,919	23,028	20,223
Transportation Costs	0	0	0	0
Other Segment Items	0	0	0	0
Adjusted EBITDA	12,152	16,821	40,547	40,065
Depreciation, Depletion and Amortization	(1,398)	(1,276)	(4,171)	(3,544)
Segment Assets	92,521	87,602	92,521	87,602
Depreciation, Depletion and Amortization	1,398	1,276	4,171	3,544
Capital Expenditures	3,104	2,805	5,884	7,605
Baltimore Marine Terminal | Intersegment Revenues
Segment Reporting Information [Line Items]
Total Revenue from Contracts with Customers	$ (14,187)	$ (16,250)	$ (48,488)	$ (37,599)