Property, Plant, Equipment and Mine Development (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Line Items]
Less: Accumulated depreciation, depletion and amortization	$ (1,551.9)	$ (1,480.5)
Property, plant, equipment and mine development, net	2,903.3	2,950.6
Land and coal interests
Property, Plant and Equipment [Line Items]
Property, Plant, Equipment, and mine development, gross	2,494.1	2,494.1
Buildings and improvements
Property, Plant and Equipment [Line Items]
Property, Plant, Equipment, and mine development, gross	590.2	550.8
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, Plant, Equipment, and mine development, gross	$ 1,370.9	$ 1,386.2