Property, Plant, Equipment and Mine Development (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Less: Accumulated depreciation, depletion and amortization	$ (2,010.9)	$ (1,787.0)
Property, plant, equipment and mine development, net	2,805.5	2,865.0
Land and coal interests
Property, Plant and Equipment [Line Items]
Property, Plant, Equipment, and mine development, gross	2,511.9	2,514.7
Buildings and improvements
Property, Plant and Equipment [Line Items]
Property, Plant, Equipment, and mine development, gross	633.5	594.2
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, Plant, Equipment, and mine development, gross	$ 1,671.0	$ 1,543.1