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Derivatives and Fair Value Measurements - Textual (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2025
USD ($)
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Sep. 30, 2026
AUD ($)
$ / $
Dec. 31, 2025
AUD ($)
Derivatives, Fair Value [Line Items]          
Revenue from physical sale of coal $ 3,861.5 $ 4,236.7 $ 4,946.7    
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) 2.7 (2.6)      
Total net assets (liabilities) | Level 2          
Derivatives, Fair Value [Line Items]          
Net financial asset (liability) 2.7 (3.6)      
Coal Trading          
Derivatives, Fair Value [Line Items]          
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings     (2.0)    
Designated as Hedging Instrument          
Derivatives, Fair Value [Line Items]          
Total Unrealized gain (loss) recognized in income on derivatives 6.0 (9.0) 154.9    
Foreign currency option contracts          
Derivatives, Fair Value [Line Items]          
Derivative, Notional Amount         $ 554,000,000.0
Derivative Asset, Notional Amount         550,000,000.0
Foreign currency option contracts | Total net assets (liabilities)          
Derivatives, Fair Value [Line Items]          
Net financial asset (liability) 2.7 (3.6)      
Foreign currency option contracts | Total net assets (liabilities) | Level 2          
Derivatives, Fair Value [Line Items]          
Net financial asset (liability) 2.7 3.6      
Foreign currency option contracts | Designated as Hedging Instrument          
Derivatives, Fair Value [Line Items]          
Total Unrealized gain (loss) recognized in income on derivatives $ 6.0 (9.0) 7.4    
Financial trading contracts | Designated as Hedging Instrument          
Derivatives, Fair Value [Line Items]          
Total Unrealized gain (loss) recognized in income on derivatives     (11.5)    
Forward Contracts | Designated as Hedging Instrument          
Derivatives, Fair Value [Line Items]          
Other Comprehensive Income (Loss), Cash Flow Hedge, Reclassification for Discontinuance, before Tax   $ 0.0      
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    
Scenario, Forecast | Foreign currency option contracts | Minimum          
Derivatives, Fair Value [Line Items]          
Derivative, Exchange Rate Cap | $ / $       0.70  
Derivative, Floor Price       0.60  
Derivative, Cap Price       0.69  
Scenario, Forecast | Foreign currency option contracts | Maximum          
Derivatives, Fair Value [Line Items]          
Derivative, Exchange Rate Cap | $ / $       0.72  
Derivative, Floor Price       0.61  
Derivative, Cap Price       0.70