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Property, Plant, Equipment and Mine Development (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Property, Plant and Equipment [Line Items]    
Less: Accumulated depreciation, depletion and amortization $ (1,460.6) $ (1,321.3)
Property, plant, equipment and mine development, net 3,008.3 3,051.1
Land and coal interests    
Property, Plant and Equipment [Line Items]    
Property, Plant, Equipment, and mine development, gross 2,484.1 2,482.9
Buildings and improvements    
Property, Plant and Equipment [Line Items]    
Property, Plant, Equipment, and mine development, gross 523.3 481.0
Machinery and equipment    
Property, Plant and Equipment [Line Items]    
Property, Plant, Equipment, and mine development, gross $ 1,461.5 $ 1,408.5