Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation of Revenue [Line Items]
Revenues	$ 1,268.8	$ 1,321.9	$ 2,632.8	$ 2,013.3
Other Income	3.8	3.4	25.6	5.6
Designated as Hedging Instrument
Disaggregation of Revenue [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	29.3	(31.4)	128.5	(318.3)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(19.3)	(122.9)	(55.5)	(192.6)
Derivative, Gain (Loss) on Derivative, Net	10.0	(154.3)	73.0	(510.9)
Derivative contracts related to forecasted sales | Designated as Hedging Instrument
Disaggregation of Revenue [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	40.3	(24.5)	159.0	(325.5)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(30.3)	(122.6)	(80.9)	(190.6)
Derivative, Gain (Loss) on Derivative, Net	10.0	(147.1)	78.1	(516.1)
Financial trading contracts | Designated as Hedging Instrument
Disaggregation of Revenue [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	(13.8)	(0.6)	(31.1)	10.2
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	13.9	0.9	31.2	0.2
Derivative, Gain (Loss) on Derivative, Net	0.1	0.3	0.1	10.4
Coal Contract and Physical commodity purchase / sale contracts | Designated as Hedging Instrument
Disaggregation of Revenue [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	40.3	(24.5)	159.0	(325.5)
Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	857.4	808.4	1,756.6	1,510.7
Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	371.6	533.3	658.8	851.3
Other (2)
Disaggregation of Revenue [Line Items]
Revenues	39.8	(19.8)	217.4	(348.7)
Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	489.9	492.5	1,080.3	984.3
Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	367.5	315.9	676.3	526.4
Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	371.6	533.3	658.8	851.3
Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenues	399.5	354.9	746.0	606.1
Seaborne Thermal | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	399.4	354.7	745.7	605.6
Seaborne Thermal | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Seaborne Thermal | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	0.1	0.2	0.3	0.5
Seaborne Thermal | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	31.9	39.8	69.4	80.2
Seaborne Thermal | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	367.5	314.9	676.3	525.4
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	372.5	533.8	660.9	855.1
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	371.6	533.3	658.8	851.3
Seaborne Metallurgical | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	0.9	0.5	2.1	3.8
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	371.6	533.3	658.8	851.3
Powder River Basin
Disaggregation of Revenue [Line Items]
Revenues	259.7	229.7	565.0	480.9
Powder River Basin | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	259.8	229.9	564.8	481.4
Powder River Basin | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Powder River Basin | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	(0.1)	(0.2)	0.2	(0.5)
Powder River Basin | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	259.8	229.9	564.8	481.4
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	199.9	224.9	449.3	428.0
Other U.S. Thermal | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	198.2	223.8	446.1	423.7
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 (2)
Disaggregation of Revenue [Line Items]
Revenues	1.7	1.1	3.2	4.3
Other U.S. Thermal | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	198.2	222.8	446.1	422.7
Other U.S. Thermal | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	1.0	0.0	1.0
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	37.2	(21.4)	211.6	(356.8)
Corporate and Other (1) | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Corporate and Other (1) | Trading and Brokerage Coal Deliveries
Disaggregation of Revenue [Line Items]
Revenues	23.3	122.0	107.8	143.3
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 (2)
Disaggregation of Revenue [Line Items]
Revenues	37.2	(21.4)	211.6	(356.8)
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