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Property, Plant, Equipment and Mine Development - Textuals (Details) - Successor - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Property, Plant, Equipment and Mine Development, Net [Line Items]    
Land and coal interests $ 3,899.9 $ 3,890.5
Buildings and improvements 453.4 470.6
Machinery and equipment 1,258.6 1,149.3
Less: Accumulated depreciation, depletion and amortization (666.2) (398.5)
Property, plant, equipment and mine development, net $ 4,945.7 $ 5,111.9