Inventories - Schedule of Inventory (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Inventories [Line Items]
Materials and supplies	$ 118.9	$ 118.1
Raw coal	50.0	53.6
Saleable coal	99.6	108.5
Total	268.5	280.2
Reserve for materials and supplies
Inventories [Line Items]
Valuation Allowances and Reserves, Balance	$ 1.1	$ 0.2