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Property, Plant, Equipment and Mine Development (Details) - USD ($)
$ in Millions
Mar. 31, 2023
Dec. 31, 2022
Property, Plant and Equipment [Line Items]    
Less: Accumulated depreciation, depletion and amortization $ (1,862.7) $ (1,787.0)
Property, plant, equipment and mine development, net 2,847.9 2,865.0
Land and coal interests    
Property, Plant and Equipment [Line Items]    
Property, Plant, Equipment, and mine development, gross 2,514.8 2,514.7
Buildings and improvements    
Property, Plant and Equipment [Line Items]    
Property, Plant, Equipment, and mine development, gross 617.3 594.2
Machinery and equipment    
Property, Plant and Equipment [Line Items]    
Property, Plant, Equipment, and mine development, gross $ 1,578.5 $ 1,543.1