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Derivatives and Fair Value Measurements - Textual (Details) $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Sep. 30, 2024 AUD ($)	Jun. 30, 2024 $ / $	Dec. 31, 2023 AUD ($)
Derivatives, Fair Value [Line Items]
Derivative, Collateral, Right to Reclaim Cash	$ 0.0	$ 255.5		$ 0.0	$ 255.5
Revenue from physical sale of coal				$ 4,946.7	4,981.9	$ 3,318.3
Unobservable Measurement Input, Uncertainty, Description				10
Fair Value, Inputs, Level 1, 2 and 3
Derivatives, Fair Value [Line Items]
Net financial asset (liability)	0.0			$ 0.0
Total net assets (liabilities)
Derivatives, Fair Value [Line Items]
Net financial asset (liability)	6.6	(192.5)		6.6	(192.5)
Coal Trading
Derivatives, Fair Value [Line Items]
Derivative, Collateral, Right to Reclaim Cash		57.5			57.5
Purchases				0.0	0.2	0.0
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Total Unrealized gain (loss) recognized in income on derivatives				154.9	(28.5)	(121.1)
Designated as Hedging Instrument \| Net Amount
Derivatives, Fair Value [Line Items]
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		198.0			198.0
Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount									$ 456,000,000
Foreign currency option contracts \| Subsequent Event
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount							$ 483,000,000
Foreign currency option contracts \| Total net assets (liabilities)
Derivatives, Fair Value [Line Items]
Net financial asset (liability)	6.2	3.0		6.2	3.0
Foreign currency option contracts \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Total Unrealized gain (loss) recognized in income on derivatives				7.4	(2.3)	(7.5)
Diesel Fuel Hedge Contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount									0
Financial trading contracts \| Total net assets (liabilities)
Derivatives, Fair Value [Line Items]
Net financial asset (liability)	0.0	11.7		0.0	11.7
Financial trading contracts \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Total Unrealized gain (loss) recognized in income on derivatives				(11.5)	9.6	1.5
Gain (Loss) on Derivative Instruments \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Total Unrealized gain (loss) recognized in income on derivatives		(65.0)	$ (86.0)
Forward Contracts \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Total Unrealized gain (loss) recognized in income on derivatives	0.0	(29.0)				(29.0)
Coal derivative contracts related to forecasted sales
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount									$ 0
Derivative contracts related to forecasted sales \| Total net assets (liabilities)
Derivatives, Fair Value [Line Items]
Net financial asset (liability)	$ 0.0	$ (209.7)		0.0	(209.7)
Derivative contracts related to forecasted sales \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Total Unrealized gain (loss) recognized in income on derivatives				$ 159.0	$ (35.8)	$ (115.1)
Scenario, Forecast \| Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, Floor Price							0.59
Derivative, Cap Price							0.72
Scenario, Forecast \| Foreign currency option contracts \| Minimum
Derivatives, Fair Value [Line Items]
Derivative, Exchange Rate Cap \| $ / $								0.69
Scenario, Forecast \| Foreign currency option contracts \| Maximum
Derivatives, Fair Value [Line Items]
Derivative, Exchange Rate Cap \| $ / $								0.72