Property, Plant, Equipment and Mine Development (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Less: Accumulated depreciation, depletion and amortization	$ (1,252.5)	$ (1,409.8)
Property, plant, equipment and mine development, net	3,152.3	4,679.1
Land and coal interests
Property, Plant and Equipment [Line Items]
Property, plant equipment and mine development, gross	2,578.8	4,022.4
Buildings and improvements
Property, Plant and Equipment [Line Items]
Property, plant equipment and mine development, gross	463.0	547.9
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant equipment and mine development, gross	$ 1,363.0	$ 1,518.6