Property, Plant, Equipment and Mine Development - (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Property, Plant, Equipment and Mine Development[Line Items]
Less: Accumulated depreciation, depletion and amortization	$ (1,321.3)	$ (1,409.8)
Property, plant, equipment and mine development, net	3,051.1	4,679.1
Coal reserves	1,500.0	2,800.0
Coal reserves not subject to depletion	100.0	100.0
Land and coal interests
Property, Plant, Equipment and Mine Development[Line Items]
Property, plant equipment and mine development, gross	2,482.9	4,022.4
Buildings and improvements
Property, Plant, Equipment and Mine Development[Line Items]
Property, plant equipment and mine development, gross	481.0	547.9
Machinery and equipment
Property, Plant, Equipment and Mine Development[Line Items]
Property, plant equipment and mine development, gross	1,408.5	1,518.6
Mining Properties and Mineral Rights
Property, Plant, Equipment and Mine Development[Line Items]
Mineral rights and advanced royalties	800.0	2,000.0
Coal Reserves Held by Fee Ownership
Property, Plant, Equipment and Mine Development[Line Items]
Coal reserves	$ 700.0	$ 800.0