Property, Plant, Equipment and Mine Development (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Successor
Property, Plant, Equipment and Mine Development, Net [Line Items]
Less: Accumulated depreciation, depletion and amortization	$ (123.8)
Total, net	5,214.2
Successor | Land and coal interests
Property, Plant, Equipment and Mine Development, Net [Line Items]
Property, plant, equipment and mine development, gross	3,785.8
Successor | Buildings and improvements
Property, Plant, Equipment and Mine Development, Net [Line Items]
Property, plant, equipment and mine development, gross	467.3
Successor | Machinery and equipment
Property, Plant, Equipment and Mine Development, Net [Line Items]
Property, plant, equipment and mine development, gross	$ 1,084.9
Predecessor
Property, Plant, Equipment and Mine Development, Net [Line Items]
Less: Accumulated depreciation, depletion and amortization		$ (5,191.9)
Total, net		8,776.7
Predecessor | Land and coal interests
Property, Plant, Equipment and Mine Development, Net [Line Items]
Property, plant, equipment and mine development, gross		10,330.8
Predecessor | Buildings and improvements
Property, Plant, Equipment and Mine Development, Net [Line Items]
Property, plant, equipment and mine development, gross		1,507.6
Predecessor | Machinery and equipment
Property, Plant, Equipment and Mine Development, Net [Line Items]
Property, plant, equipment and mine development, gross		$ 2,130.2