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Property, Plant, Equipment and Mine Development - (Details) - USD ($)
$ in Millions
Dec. 31, 2025
Dec. 31, 2024
Property, Plant, Equipment and Mine Development[Line Items]    
Less: Accumulated depreciation, depletion and amortization $ (2,673.8) $ (2,372.5)
Property, plant, equipment and mine development, net 3,153.3 3,081.5
Coal reserves 1,100.0 1,100.0
Coal reserves not subject to depletion 100.0 100.0
Other U.S. Thermal    
Property, Plant, Equipment and Mine Development[Line Items]    
At-risk assets 64.0  
Land and coal interests    
Property, Plant, Equipment and Mine Development[Line Items]    
Property, plant equipment and mine development, gross 2,691.2 2,648.5
Buildings and improvements    
Property, Plant, Equipment and Mine Development[Line Items]    
Property, plant equipment and mine development, gross 758.8 726.7
Machinery and equipment    
Property, Plant, Equipment and Mine Development[Line Items]    
Property, plant equipment and mine development, gross 2,377.1 2,078.8
Coal Reserves Held by Fee Ownership    
Property, Plant, Equipment and Mine Development[Line Items]    
Coal reserves 500.0 500.0
Coal reserves and resources comprised of mineral rights for leased coal interests and advance royalties    
Property, Plant, Equipment and Mine Development[Line Items]    
Coal reserves $ 600.0 $ 600.0