Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	$ 4,236.7	$ 4,946.7	$ 4,981.9
Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,213.9	1,329.7	1,345.6
Seaborne Metallurgical
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,055.6	1,301.9	1,616.9
Powder River Basin
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,098.8	1,198.1	1,065.5
Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	822.6	888.2	952.2
Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	45.8	228.8	1.7
Thermal coal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	3,092.8	3,390.5	3,358.3
Thermal coal \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,211.4	1,328.9	1,344.9
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	1,096.4	1,193.9	1,066.0
Thermal coal \| Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	785.0	867.7	947.4
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	2,032.2	2,198.0	2,177.5
Thermal coal \| Domestic \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	150.8	136.4	167.6
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	1,096.4	1,193.9	1,066.0
Thermal coal \| Domestic \| Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	785.0	867.7	943.9
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	1,060.6	1,192.5	1,180.8
Thermal coal \| Export \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,060.6	1,192.5	1,177.3
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	3.5
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	1,049.5	1,299.6	1,610.8
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	1,049.5	1,299.6	1,610.8
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	1,049.5	1,299.6	1,610.8
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	1,049.5	1,299.6	1,610.8
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 (2)
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	94.4	256.6	12.8
Other (2) \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	2.5	0.8	0.7
Other (2) \| Seaborne Metallurgical
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	6.1	2.3	6.1
Other (2) \| Powder River Basin
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	2.4	4.2	(0.5)
Other (2) \| Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	37.6	20.5	4.8
Other (2) \| Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	$ 45.8	$ 228.8	$ 1.7