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Property, Plant, Equipment and Mine Development (Details) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Property, Plant and Equipment [Abstract]    
Land and coal interests $ 4,164.3 $ 4,148.8
Buildings and improvements 549.1 559.3
Machinery and equipment 1,528.7 1,456.3
Less: Accumulated depreciation, depletion and amortization (1,267.3) (957.4)
Property, plant, equipment and mine development, net $ 4,974.8 $ 5,207.0