Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Disaggregation of Revenue [Line Items]
Revenues	$ 1,012.1	$ 1,088.0	$ 2,839.2	$ 3,113.6
Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	735.8	829.4	2,070.8	2,274.4
Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	258.5	240.4	730.1	778.4
Other
Disaggregation of Revenue [Line Items]
Revenues	17.8	18.2	38.3	60.8
Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	527.0	557.0	1,475.4	1,488.3
Export \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	208.8	272.4	595.4	786.1
Export \| Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	258.5	240.4	730.1	778.4
Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenues	242.7	313.2	702.9	904.6
Seaborne Thermal \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	242.4	311.2	702.5	902.4
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	0.3	2.0	0.4	2.2
Seaborne Thermal \| Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	33.6	38.8	107.1	116.3
Seaborne Thermal \| Export \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	208.8	272.4	595.4	786.1
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	258.9	242.5	731.2	783.8
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	258.5	240.4	730.1	778.4
Seaborne Metallurgical \| Other
Disaggregation of Revenue [Line Items]
Revenues	0.4	2.1	1.1	5.4
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	258.5	240.4	730.1	778.4
Powder River Basin
Disaggregation of Revenue [Line Items]
Revenues	301.4	305.3	852.7	781.3
Powder River Basin \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	301.4	304.2	852.6	779.7
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	1.1	0.1	1.6
Powder River Basin \| Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	301.4	304.2	852.6	779.7
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	192.0	216.7	515.8	610.3
Other U.S. Thermal \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	192.0	214.0	515.7	592.3
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	0.0	2.7	0.1	18.0
Other U.S. Thermal \| Domestic \| Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	192.0	214.0	515.7	592.3
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	17.1	10.3	36.6	33.6
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	17.1	10.3	36.6	33.6
Corporate and Other (1) \| Trading and Brokerage Coal Deliveries
Disaggregation of Revenue [Line Items]
Revenues	14.0	5.9	25.8	20.1
Corporate and Other (1) \| Other revenue
Disaggregation of Revenue [Line Items]
Revenues	3.1	4.4	10.8	13.5
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