Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Disaggregation of Revenue [Line Items]
Revenues	$ 890.1	$ 1,042.0	$ 1,827.1	$ 2,025.6
Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	625.8	729.2	1,335.0	1,445.0
Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	251.9	294.0	471.6	538.0
Other
Disaggregation of Revenue [Line Items]
Revenues	12.4	18.8	20.5	42.6
Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	466.5	458.6	948.4	931.3
Export \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	159.3	270.6	386.6	513.7
Export \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	251.9	294.0	471.6	538.0
Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenues	195.1	307.5	460.2	591.4
Seaborne Thermal \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	195.1	307.5	460.1	591.2
Seaborne Thermal \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Seaborne Thermal \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.1	0.2
Seaborne Thermal \| Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	35.8	36.9	73.5	77.5
Seaborne Thermal \| Export \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	159.3	270.6	386.6	513.7
Seaborne Thermal \| Export \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Seaborne Metallurgical
Disaggregation of Revenue [Line Items]
Revenues	252.2	294.3	472.3	541.3
Seaborne Metallurgical \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Seaborne Metallurgical \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	251.9	294.0	471.6	538.0
Seaborne Metallurgical \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.3	0.3	0.7	3.3
Seaborne Metallurgical \| Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Seaborne Metallurgical \| Export \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Seaborne Metallurgical \| Export \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	251.9	294.0	471.6	538.0
Powder River Basin
Disaggregation of Revenue [Line Items]
Revenues	275.7	221.9	551.3	476.0
Powder River Basin \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	275.7	221.4	551.2	475.5
Powder River Basin \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Powder River Basin \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.5	0.1	0.5
Powder River Basin \| Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	275.7	221.4	551.2	475.5
Powder River Basin \| Export \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Powder River Basin \| Export \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Other U.S. Thermal
Disaggregation of Revenue [Line Items]
Revenues	155.1	202.0	323.8	393.6
Other U.S. Thermal \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	155.0	200.3	323.7	378.3
Other U.S. Thermal \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Other U.S. Thermal \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.1	1.7	0.1	15.3
Other U.S. Thermal \| Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	155.0	200.3	323.7	378.3
Other U.S. Thermal \| Export \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Other U.S. Thermal \| Export \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenues	12.0	16.3	19.5	23.3
Corporate and Other (1) \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other (1) \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other (1) \| Other
Disaggregation of Revenue [Line Items]
Revenues	12.0	16.3	19.5	23.3
Corporate and Other (1) \| Trading and Brokerage Coal Deliveries
Disaggregation of Revenue [Line Items]
Revenues	7.8	12.1	11.8	14.2
Corporate and Other (1) \| Other revenue
Disaggregation of Revenue [Line Items]
Revenues	4.2	4.2	7.7	9.1
Corporate and Other (1) \| Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other (1) \| Export \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other (1) \| Export \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0