Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation of Revenue [Line Items]
Revenues	$ 1,078.9	$ 1,342.5	$ 3,711.7	$ 3,355.8
Other Income	3.8	2.7	29.4	8.3
Designated as Hedging Instrument
Disaggregation of Revenue [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	19.1	88.8	147.6	(229.4)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(21.9)	(118.5)	(77.3)	(311.2)
Derivative, Gain (Loss) on Derivative, Net	(2.8)	(29.7)	70.3	(540.6)
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	0.0	90.4	159.0	(235.1)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	0.0	(117.4)	(80.9)	(308.0)
Derivative, Gain (Loss) on Derivative, Net		(27.0)	78.1	(543.1)
Financial trading contracts | Designated as Hedging Instrument
Disaggregation of Revenue [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	19.6	(0.2)	(11.5)	10.1
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(19.7)	0.5	11.5	0.6
Derivative, Gain (Loss) on Derivative, Net	(0.1)	0.3	0.0	10.7
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	0.0	90.4	159.0	(235.1)
Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	837.6	903.1	2,594.2	2,413.8
Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	246.0	309.9	904.8	1,161.2
Other (2)
Disaggregation of Revenue [Line Items]
Revenues	(4.7)	129.5	212.7	(219.2)
Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	571.7	594.8	1,652.0	1,579.1
Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	265.9	308.3	942.2	834.7
Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues	246.0	309.9	904.8	1,161.2
Seaborne Thermal
Disaggregation of Revenue [Line Items]
Revenues	297.4	353.2	1,043.4	959.3
Seaborne Thermal | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	297.2	353.1	1,042.9	958.7
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	0.2	0.1	0.5	0.6
Seaborne Thermal | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	31.3	44.8	100.7	125.0
Seaborne Thermal | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	265.9	308.3	942.2	833.7
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	247.0	310.7	907.9	1,165.8
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	246.0	309.9	904.8	1,161.2
Seaborne Metallurgical | Other (2)
Disaggregation of Revenue [Line Items]
Revenues	1.0	0.8	3.1	4.6
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	246.0	309.9	904.8	1,161.2
Powder River Basin
Disaggregation of Revenue [Line Items]
Revenues	313.0	290.5	878.0	771.4
Powder River Basin | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	313.3	290.2	878.1	771.6
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	(0.3)	0.3	(0.1)	(0.2)
Powder River Basin | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	313.3	290.2	878.1	771.6
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	228.2	261.4	677.5	689.4
Other U.S. Thermal | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	227.1	259.8	673.2	683.5
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	1.1	1.6	4.3	5.9
Other U.S. Thermal | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	227.1	259.8	673.2	682.5
Other U.S. Thermal | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	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	(6.7)	126.7	204.9	(230.1)
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	(10.4)	150.7	97.4	294.0
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 (2)
Disaggregation of Revenue [Line Items]
Revenues	(6.7)	126.7	204.9	(230.1)
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