Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	$ 4,946.7	$ 4,981.9	$ 3,318.3
Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,329.7	1,345.6	934.0
Seaborne Metallurgical
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,301.9	1,616.9	727.7
Powder River Basin
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,198.1	1,065.5	971.2
Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	888.2	952.2	689.1
Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	228.8	1.7	(3.7)
Thermal coal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	3,390.5	3,358.3	2,583.1
Thermal coal \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,328.9	1,344.9	932.5
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	1,193.9	1,066.0	970.7
Thermal coal \| Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	867.7	947.4	679.9
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	2,198.0	2,177.5	1,814.1
Thermal coal \| Domestic \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	136.4	167.6	173.5
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	1,193.9	1,066.0	970.7
Thermal coal \| Domestic \| Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	867.7	943.9	669.9
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	1,192.5	1,180.8	769.0
Thermal coal \| Export \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	1,192.5	1,177.3	759.0
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	3.5	10.0
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	1,299.6	1,610.8	719.8
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	1,299.6	1,610.8	719.8
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	1,299.6	1,610.8	719.8
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	1,299.6	1,610.8	719.8
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	256.6	12.8	15.4
Other (2) \| Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	0.8	0.7	1.5
Other (2) \| Seaborne Metallurgical
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	2.3	6.1	7.9
Other (2) \| Powder River Basin
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	4.2	(0.5)	0.5
Other (2) \| Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	20.5	4.8	9.2
Other (2) \| Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue from physical sale of coal	$ 228.8	$ 1.7	$ (3.7)