Segment Information - Schedule of Segment Reporting Information by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
Segment Reporting Information [Line Items]
Revenues	$ 1,084,278	$ 1,017,406
Adjusted EBITDA	208,459	165,305
Reconciliation of Segment Profit or Loss Measure to Consolidated Earnings (Loss) Before Income Tax:
Other Profit or Loss	1,389	368
Depreciation, Depletion and Amortization	(146,295)	(121,556)
General and Administrative Costs	(36,079)	(89,323)
Interest Expense	(11,192)	(8,019)
Interest Income	4,748	6,318
Loss on Debt Extinguishment	0	(11,680)
Non-Service Related Pension and Postretirement Benefit Costs	(5,653)	(6,202)
Closed and Idle Mine Costs	(4,480)	(4,644)
Other Operating Income and Expense, net	9,995	9,859
Other Costs	(275)	(13,919)
Earnings (Loss) Before Income Tax	20,617	(73,493)
Additional Information
Segment Assets	6,055,921	6,251,946	$ 6,130,053
Depreciation, Depletion and Amortization	146,295	121,556
Capital Expenditures	73,097	64,822
Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,094,559	1,030,481
Intersegment Revenues
Segment Reporting Information [Line Items]
Revenues	(15,710)	(16,270)
Corporate and Other
Segment Reporting Information [Line Items]
Revenues	5,429	3,195
Reconciliation of Segment Profit or Loss Measure to Consolidated Earnings (Loss) Before Income Tax:
Depreciation, Depletion and Amortization	(12,979)	(12,218)
Additional Information
Segment Assets	1,615,261	1,841,276
Depreciation, Depletion and Amortization	12,979	12,218
Capital Expenditures	5,725	3,125
High CV Thermal
Segment Reporting Information [Line Items]
Revenues	552,832	542,086
High CV Thermal | Operating Segments
Segment Reporting Information [Line Items]
Revenues	552,832	542,086
Cash Costs of Revenue	327,825	303,561
Transportation Costs	99,373	93,729
Other Segment Items	0	0
Adjusted EBITDA	125,634	144,796
Reconciliation of Segment Profit or Loss Measure to Consolidated Earnings (Loss) Before Income Tax:
Depreciation, Depletion and Amortization	(52,676)	(51,290)
Additional Information
Segment Assets	2,117,031	2,252,607
Depreciation, Depletion and Amortization	52,676	51,290
Capital Expenditures	50,332	33,995
High CV Thermal | Intersegment Revenues
Segment Reporting Information [Line Items]
Revenues	0	0
Metallurgical
Segment Reporting Information [Line Items]
Revenues	342,335	304,580
Metallurgical | Operating Segments
Segment Reporting Information [Line Items]
Revenues	342,335	304,580
Cash Costs of Revenue	226,305	210,775
Transportation Costs	67,787	76,982
Other Segment Items	(9,723)	36,406
Adjusted EBITDA	57,966	(19,583)
Reconciliation of Segment Profit or Loss Measure to Consolidated Earnings (Loss) Before Income Tax:
Depreciation, Depletion and Amortization	(70,861)	(45,889)
Additional Information
Segment Assets	1,975,095	1,766,112
Depreciation, Depletion and Amortization	70,861	45,889
Capital Expenditures	13,700	23,964
Metallurgical | Intersegment Revenues
Segment Reporting Information [Line Items]
Revenues	0	0
PRB
Segment Reporting Information [Line Items]
Revenues	175,180	162,589
PRB | Operating Segments
Segment Reporting Information [Line Items]
Revenues	175,180	162,589
Cash Costs of Revenue	162,596	133,158
Transportation Costs	3,707	2,740
Other Segment Items	0	0
Adjusted EBITDA	8,877	26,691
Reconciliation of Segment Profit or Loss Measure to Consolidated Earnings (Loss) Before Income Tax:
Depreciation, Depletion and Amortization	(8,300)	(10,780)
Additional Information
Segment Assets	254,984	304,455
Depreciation, Depletion and Amortization	8,300	10,780
Capital Expenditures	751	2,496
PRB | Intersegment Revenues
Segment Reporting Information [Line Items]
Revenues	0	0
Core Marine Terminal
Segment Reporting Information [Line Items]
Revenues	8,502	4,956
Core Marine Terminal | Operating Segments
Segment Reporting Information [Line Items]
Revenues	24,212	21,226
Cash Costs of Revenue	8,230	7,825
Transportation Costs	0	0
Other Segment Items	0	0
Adjusted EBITDA	15,982	13,401
Reconciliation of Segment Profit or Loss Measure to Consolidated Earnings (Loss) Before Income Tax:
Depreciation, Depletion and Amortization	(1,479)	(1,379)
Additional Information
Segment Assets	93,550	87,496
Depreciation, Depletion and Amortization	1,479	1,379
Capital Expenditures	2,589	1,242
Core Marine Terminal | Intersegment Revenues
Segment Reporting Information [Line Items]
Revenues	$ (15,710)	$ (16,270)