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Derivatives and Fair Value Measurements - Textual (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Sep. 30, 2025 AUD ($) $ / $	Dec. 31, 2024 AUD ($)
Derivatives, Fair Value [Line Items]
Revenue from physical sale of coal	$ 4,236.7	$ 4,946.7	$ 4,981.9
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
Total net assets (liabilities)
Derivatives, Fair Value [Line Items]
Net financial asset (liability)	(2.6)	6.6
Total net assets (liabilities) | Level 2
Derivatives, Fair Value [Line Items]
Net financial asset (liability)	(3.6)	6.2
Coal Trading
Derivatives, Fair Value [Line Items]
Purchases	0.0	0.0	0.2
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Total Unrealized gain (loss) recognized in income on derivatives	(9.0)	154.9	(28.5)
Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount					$ 528,000,000
Derivative Asset, Notional Amount					388,000,000.0
Foreign currency option contracts | Total net assets (liabilities)
Derivatives, Fair Value [Line Items]
Net financial asset (liability)	(3.6)	6.2
Foreign currency option contracts | Total net assets (liabilities) | Level 2
Derivatives, Fair Value [Line Items]
Net financial asset (liability)	(3.6)	6.2
Foreign currency option contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Total Unrealized gain (loss) recognized in income on derivatives	$ (9.0)	7.4	(2.3)
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
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)
Forward Contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Total Unrealized gain (loss) recognized in income on derivatives		0.0	(29.0)
Coal derivative contracts related to forecasted sales
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount					$ 0
Derivative contracts related to forecasted sales | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Total Unrealized gain (loss) recognized in income on derivatives		$ 159.0	$ (35.8)
Scenario, Forecast | Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, Floor Price				0.59
Derivative, Cap Price				0.73
Scenario, Forecast | Foreign currency option contracts | Minimum
Derivatives, Fair Value [Line Items]
Derivative, Exchange Rate Cap | $ / $				0.70
Scenario, Forecast | Foreign currency option contracts | Maximum
Derivatives, Fair Value [Line Items]
Derivative, Exchange Rate Cap | $ / $				0.74

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Nominal or face amount used to calculate payments on the derivative asset.

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Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

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