Segment Information - Schedule of Segment Reporting Information by Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Revenues	$ 4,164,775	$ 2,164,406	$ 2,506,635
Adjusted EBITDA	675,155	746,106	1,147,080
Reconciliation of segment profit or loss measure to consolidated earnings (loss) before income tax:
Other Profit or Loss	2,940	4,731	(663)
Depreciation, Depletion and Amortization	(621,067)	(223,526)	(241,317)
General and Administrative Costs	(214,856)	(115,224)	(103,470)
Interest Expense	(40,124)	(22,192)	(29,325)
Interest Income	25,890	19,223	13,597
Loss on Debt Extinguishment	(11,680)	0	(2,725)
Non-Service Related Pension and Postretirement Benefit Costs	(25,728)	(17,384)	(7,011)
Idle Mine Costs	(24,145)	(4,859)	(4,515)
Other Operating Income and Expense, net	33,904	(43,224)	19,111
Other Costs	(33,992)	(13,004)	(12,890)
(Loss) Earnings Before Income Tax	(233,703)	330,647	777,872
Additional Information
Segment Assets	6,130,053	2,879,543	2,675,003
Depreciation, Depletion and Amortization	621,067	223,526	241,317
Capital Expenditures	284,581	177,988	167,791
Operating Segments
Segment Reporting Information [Line Items]
Revenues	4,217,161	2,205,380	2,564,901
Intersegment Revenues
Segment Reporting Information [Line Items]
Revenues	(66,203)	(56,682)	(58,266)
Corporate and Other
Segment Reporting Information [Line Items]
Revenues	13,817	15,708
Reconciliation of segment profit or loss measure to consolidated earnings (loss) before income tax:
Depreciation, Depletion and Amortization	(121,496)	(37,005)	(37,464)
Additional Information
Segment Assets	1,648,510	910,807	882,382
Depreciation, Depletion and Amortization	121,496	37,005	37,464
Capital Expenditures	21,722	7,003	6,017
High CV Thermal
Segment Reporting Information [Line Items]
Revenues	2,208,643	2,004,567	2,359,796
High CV Thermal | Operating Segments
Segment Reporting Information [Line Items]
Revenues	2,208,643	2,004,567	2,359,796
Cash Costs of Revenue	1,252,525	973,139	939,892
Transportation Costs	364,888	321,367	335,186
Other Segment Items	11,124	0	(429)
Adjusted EBITDA	580,106	710,061	1,085,147
Reconciliation of segment profit or loss measure to consolidated earnings (loss) before income tax:
Depreciation, Depletion and Amortization	(207,891)	(172,997)	(191,391)
Additional Information
Segment Assets	2,107,970	1,739,792	1,582,434
Depreciation, Depletion and Amortization	207,891	172,997	191,391
Capital Expenditures	174,153	149,021	144,550
High CV Thermal | Intersegment Revenues
Segment Reporting Information [Line Items]
Revenues	0	0	0
Metallurgical
Segment Reporting Information [Line Items]
Revenues	1,202,055	113,067	98,939
Metallurgical | Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,202,055	113,067	98,939
Cash Costs of Revenue	868,839	128,445	98,803
Transportation Costs	276,935	9,341	17,183
Other Segment Items	81,936	(390)	(73)
Adjusted EBITDA	(25,655)	(24,329)	(16,974)
Reconciliation of segment profit or loss measure to consolidated earnings (loss) before income tax:
Depreciation, Depletion and Amortization	(252,882)	(8,635)	(8,235)
Additional Information
Segment Assets	2,020,450	140,798	126,865
Depreciation, Depletion and Amortization	252,882	8,635	8,235
Capital Expenditures	73,107	13,614	12,656
Metallurgical | Intersegment Revenues
Segment Reporting Information [Line Items]
Revenues	0	0	0
PRB
Segment Reporting Information [Line Items]
Revenues	718,783	0	0
PRB | Operating Segments
Segment Reporting Information [Line Items]
Revenues	718,783	0	0
Cash Costs of Revenue	643,601	0	0
Transportation Costs	11,317	0	0
Other Segment Items	0	0	0
Adjusted EBITDA	63,865	0	0
Reconciliation of segment profit or loss measure to consolidated earnings (loss) before income tax:
Depreciation, Depletion and Amortization	(33,177)	0	0
Additional Information
Segment Assets	261,193	0	0
Depreciation, Depletion and Amortization	33,177	0	0
Capital Expenditures	8,126	0	0
PRB | Intersegment Revenues
Segment Reporting Information [Line Items]
Revenues	0	0	0
Core Marine Terminal
Segment Reporting Information [Line Items]
Revenues	21,477	31,064	47,900
Core Marine Terminal | Operating Segments
Segment Reporting Information [Line Items]
Revenues	87,680	87,746	106,166
Cash Costs of Revenue	30,841	27,372	27,259
Transportation Costs	0	0	0
Other Segment Items	0	0	0
Adjusted EBITDA	56,839	60,374	78,907
Reconciliation of segment profit or loss measure to consolidated earnings (loss) before income tax:
Depreciation, Depletion and Amortization	(5,621)	(4,889)	(4,227)
Additional Information
Segment Assets	91,930	88,146	83,322
Depreciation, Depletion and Amortization	5,621	4,889	4,227
Capital Expenditures	7,473	8,350	4,568
Core Marine Terminal | Intersegment Revenues
Segment Reporting Information [Line Items]
Revenues	$ (66,203)	$ (56,682)	$ (58,266)