Revenue Recognition - Revenue by Product Type and Market (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Disaggregation of Revenue [Line Items]
Revenue	$ 1,364.0	$ 691.4
Other Income	21.8	4.4
Designated as Hedging Instrument
Disaggregation of Revenue [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	99.2	(286.9)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(36.1)	(69.7)
Derivative, Gain (Loss) on Derivative, Net	63.1	(356.6)
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	118.7	(301.0)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(50.6)	(68.0)
Derivative, Gain (Loss) on Derivative, Net	68.1	(369.0)
Financial trading contracts | Designated as Hedging Instrument
Disaggregation of Revenue [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	(17.3)	10.8
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	17.3	(0.7)
Derivative, Gain (Loss) on Derivative, Net	0.0	10.1
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	118.7	(301.0)
Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	899.2	702.3
Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	287.2	318.0
Other (2)
Disaggregation of Revenue [Line Items]
Revenue	177.6	(328.9)
Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	590.4	491.8
Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	308.8	210.5
Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	287.2	318.0
Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenue	346.5	251.2
Seaborne Thermal Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	346.3	250.9
Seaborne Thermal Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Seaborne Thermal Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenue	0.2	0.3
Seaborne Thermal Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	37.5	40.4
Seaborne Thermal Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	308.8	210.5
Seaborne Thermal Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenue	288.4	321.3
Seaborne Metallurgical Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Seaborne Metallurgical Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	287.2	318.0
Seaborne Metallurgical Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenue	1.2	3.3
Seaborne Metallurgical Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Seaborne Metallurgical Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Seaborne Metallurgical Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	287.2	318.0
Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenue	305.3	251.2
Powder River Basin Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	305.0	251.5
Powder River Basin Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Powder River Basin Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenue	0.3	(0.3)
Powder River Basin Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	305.0	251.5
Powder River Basin Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Powder River Basin Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Other U.S. Thermal Mining
Disaggregation of Revenue [Line Items]
Revenue	249.4	203.1
Other U.S. Thermal Mining | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	247.9	199.9
Other U.S. Thermal Mining | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Other U.S. Thermal Mining | Other (2)
Disaggregation of Revenue [Line Items]
Revenue	1.5	3.2
Other U.S. Thermal Mining | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	247.9	199.9
Other U.S. Thermal Mining | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Other U.S. Thermal Mining | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Corporate and Other (1)
Disaggregation of Revenue [Line Items]
Revenue	174.4	(335.4)
Corporate and Other (1) | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Corporate and Other (1) | Trading and Brokerage Coal Deliveries
Disaggregation of Revenue [Line Items]
Revenue	84.5	19.1
Corporate and Other (1) | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Corporate and Other (1) | Other (2)
Disaggregation of Revenue [Line Items]
Revenue	174.4	(335.4)
Corporate and Other (1) | Domestic | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Corporate and Other (1) | Export | Thermal coal
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0
Corporate and Other (1) | Export | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenue	$ 0.0	$ 0.0