Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	$ 3,861.5	$ 4,236.7	$ 4,946.7
Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	908.5	1,213.9	1,329.7
Seaborne Metallurgical
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,036.6	1,055.6	1,301.9
Powder River Basin
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,153.0	1,098.8	1,198.1
Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	707.3	822.6	888.2
Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	56.1	45.8	228.8
Thermal coal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	2,768.3	3,092.8	3,390.5
Thermal coal \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	907.8	1,211.4	1,328.9
Thermal coal \| Seaborne Metallurgical
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Thermal coal \| Powder River Basin
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,153.0	1,096.4	1,193.9
Thermal coal \| Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	707.5	785.0	867.7
Thermal coal \| Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Thermal coal \| Domestic
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,996.5	2,032.2	2,198.0
Thermal coal \| Domestic \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	136.0	150.8	136.4
Thermal coal \| Domestic \| Seaborne Metallurgical
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Thermal coal \| Domestic \| Powder River Basin
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,153.0	1,096.4	1,193.9
Thermal coal \| Domestic \| Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	707.5	785.0	867.7
Thermal coal \| Domestic \| Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Thermal coal \| Export
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	771.8	1,060.6	1,192.5
Thermal coal \| Export \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	771.8	1,060.6	1,192.5
Thermal coal \| Export \| Seaborne Metallurgical
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Thermal coal \| Export \| Powder River Basin
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Thermal coal \| Export \| Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Thermal coal \| Export \| Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,034.8	1,049.5	1,299.6
Metallurgical coal \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Metallurgical coal \| Seaborne Metallurgical
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,034.8	1,049.5	1,299.6
Metallurgical coal \| Powder River Basin
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Metallurgical coal \| Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Metallurgical coal \| Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Metallurgical coal \| Export
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,034.8	1,049.5	1,299.6
Metallurgical coal \| Export \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Metallurgical coal \| Export \| Seaborne Metallurgical
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,034.8	1,049.5	1,299.6
Metallurgical coal \| Export \| Powder River Basin
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Metallurgical coal \| Export \| Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Metallurgical coal \| Export \| Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	58.4	94.4	256.6
Other \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.7	2.5	0.8
Other \| Seaborne Metallurgical
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1.8	6.1	2.3
Other \| Powder River Basin
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.0	2.4	4.2
Other \| Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	(0.2)	37.6	20.5
Other \| Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	$ 56.1	$ 45.8	$ 228.8