Inventories (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Inventories [Line Items]
Materials and supplies	$ 101.6	$ 104.5
Raw coal	40.9	29.6
Saleable coal	108.3	69.6
Total	250.8	203.7
Reserve for materials and supplies [Member]
Inventories [Line Items]
Valuation Allowances and Reserves, Balance	$ 6.1	$ 5.6