Property, Plant, Equipment and Mine Development (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Less: Accumulated depreciation, depletion and amortization	$ (1,933.2)	$ (1,787.0)
Property, plant, equipment and mine development, net	2,819.5	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	628.8	594.2
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, Plant, Equipment, and mine development, gross	$ 1,612.0	$ 1,543.1