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Property, Plant, Equipment and Mine Development (Details) - USD ($)
$ in Millions
Sep. 30, 2024
Dec. 31, 2023
Property, Plant and Equipment [Line Items]    
Less: Accumulated depreciation, depletion and amortization $ (2,290.4) $ (2,066.3)
Property, plant, equipment and mine development, net 3,013.5 2,844.1
Land and coal interests    
Property, Plant and Equipment [Line Items]    
Property, Plant, Equipment, and mine development, gross 2,624.4 2,475.2
Buildings and improvements    
Property, Plant and Equipment [Line Items]    
Property, Plant, Equipment, and mine development, gross 658.1 647.6
Machinery and equipment    
Property, Plant and Equipment [Line Items]    
Property, Plant, Equipment, and mine development, gross $ 2,021.4 $ 1,787.6