Property, Plant, Equipment and Mine Development (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant, Equipment and Mine Development[Line Items]
Less: Accumulated depreciation, depletion and amortization	$ (1,409.8)	$ (957.4)
Property, plant, equipment and mine development, net	4,679.1	5,207.0	$ 5,111.9
Coal reserves	2,800.0	3,000.0
Coal reserves not subject to depletion	100.0	200.0
Acquired interest in mineral rights	100.0	100.0
Land and coal interests
Property, Plant, Equipment and Mine Development[Line Items]
Property, Plant and Equipment, Gross	4,022.4	4,148.8
Buildings and improvements
Property, Plant, Equipment and Mine Development[Line Items]
Property, Plant and Equipment, Gross	547.9	559.3
Machinery and equipment
Property, Plant, Equipment and Mine Development[Line Items]
Property, Plant and Equipment, Gross	1,518.6	1,456.3
Mining Properties and Mineral Rights
Property, Plant, Equipment and Mine Development[Line Items]
Mineral rights and advanced royalties	2,000.0	2,100.0
Coal Reserves Held by Fee Ownership
Property, Plant, Equipment and Mine Development[Line Items]
Coal reserves	$ 800.0	$ 900.0