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Derivatives and Fair Value Measurements - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($) $ / $	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) $ / $	Sep. 30, 2022 USD ($)	Sep. 30, 2023 AUD ($) $ / $	Dec. 31, 2022 USD ($)
Derivatives, Fair Value [Line Items]
Unobservable Measurement Input, Uncertainty, Description			10
Asset impairment	$ 0.0	$ 1.7	$ 2.0	$ 1.7
Fair Value, Inputs, Level 1, 2 and 3
Derivatives, Fair Value [Line Items]
Derivative assets (liabilities), at fair value	0.0	0.0	0.0	0.0
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Variation margin (received) posted	0.0		0.0			$ (209.7)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	19.1	88.8	147.6	(229.4)
Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Notional amount	$ 339.0		$ 339.0		$ 615,000,000
Derivative, Floor Price					0.60
Derivative, Cap Price					0.75
Foreign currency option contracts \| Minimum
Derivatives, Fair Value [Line Items]
Derivative, exchange rate cap (in dollars per share) \| $ / $	0.70		0.70		0.70
Foreign currency option contracts \| Maximum
Derivatives, Fair Value [Line Items]
Derivative, exchange rate cap (in dollars per share) \| $ / $	0.72		0.72		0.72
Foreign currency option contracts \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	$ (0.5)	(1.4)	$ 0.1	(4.4)
Financial trading contracts \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	19.6	(0.2)	(11.5)	10.1
Diesel Fuel Hedge Contracts
Derivatives, Fair Value [Line Items]
Notional amount					$ 0
Coal Contract and Physical commodity purchase / sale contracts \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	0.0	90.4	159.0	(235.1)
Gain (Loss) on Derivative Instruments \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax		49.0		257.0
Forward Contracts \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax		41.0	0.0	22.0
Derivative contracts related to forecasted sales \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	0.0	$ 90.4	159.0	$ (235.1)
Coal derivative contracts related to forecasted sales
Derivatives, Fair Value [Line Items]
Notional amount					$ 0
Variation Margin Cash \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Variation margin (received) posted						198.0
Total Margin Posted
Derivatives, Fair Value [Line Items]
Derivative Liability, Subject to Master Netting Arrangement, Collateral, Right to Reclaim Cash Not Offset	$ 0.0		$ 0.0			255.5
Initial Margin Posted
Derivatives, Fair Value [Line Items]
Derivative Liability, Subject to Master Netting Arrangement, Collateral, Right to Reclaim Cash Not Offset						$ 57.5