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Derivatives and Fair Value Measurements - Narrative (Details) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Dec. 31, 2022 t $ / $	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($) t	Sep. 30, 2021 USD ($)	Dec. 31, 2022 t $ / $	Sep. 30, 2022 AUD ($)	Dec. 31, 2021 USD ($)
Derivatives, Fair Value [Line Items]
Derivative, initial margin posted		$ 465.9		$ 465.9				$ 130.1
Proceeds from common stock issuances, net of costs				222.0	$ 177.2
Net loss (gain) on early debt extinguishment		(8.7)	$ 16.0	$ (34.5)	31.3
Unobservable Measurement Input, Uncertainty, Description				10
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset [Abstract]
Revenue		1,342.5	679.0	$ 3,355.8	2,053.7
Asset impairment		1.7	0.0	1.7	0.0
Fair Value, Inputs, Level 1, 2 and 3
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset [Abstract]
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		12.2		12.2				3.4
Unrealized (loss) gain recognized in income on derivatives		88.8	(237.8)	(229.4)	(271.5)
Designated as Hedging Instrument | Net Amount
Derivatives, Fair Value [Line Items]
Variation margin (received) posted		377.8		377.8				91.8
Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Notional amount							$ 855,000,000
Foreign currency option contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Unrealized (loss) gain recognized in income on derivatives		(1.4)	0.6	$ (4.4)	(8.3)
Financial trading contracts
Derivatives, Fair Value [Line Items]
Derivative notional amount (in tonnes) | t				1,000,000
Financial trading contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Unrealized (loss) gain recognized in income on derivatives		(0.2)	0.0	$ 10.1	0.8
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]
Unrealized (loss) gain recognized in income on derivatives		90.4	(238.4)	(235.1)	(264.0)
Gain (Loss) on Derivative Instruments | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Unrealized (loss) gain recognized in income on derivatives		49.0		(257.0)
Forward Contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Unrealized (loss) gain recognized in income on derivatives		41.0		22.0
Derivative contracts related to forecasted sales | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Unrealized (loss) gain recognized in income on derivatives		90.4	$ (238.4)	(235.1)	$ (264.0)
Forecast | Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, exchange rate floor (in dollars per share) | $ / $	0.70					0.70
Derivative, exchange rate cap (in dollars per share) | $ / $	0.80					0.80
Forecast | Financial trading contracts
Derivatives, Fair Value [Line Items]
Derivative notional amount (in tonnes) | t	100,000					900,000
Forecast | Coal Contract to settle in 2022
Derivatives, Fair Value [Line Items]
Derivative notional amount (in tonnes) | t						300,000
Forecast | Coal Contract to settle in 2023
Derivatives, Fair Value [Line Items]
Derivative notional amount (in tonnes) | t						600,000
Coal Trading
Derivatives, Fair Value [Line Items]
Derivative, initial margin posted		$ 88.1		$ 88.1				$ 38.3

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