Properties, Plants, and Equipment, Net - Components (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Components of net properties, plants and equipment
Gross investment, at cost	$ 2,296.6	$ 2,099.3
Less: Accumulated depreciation	(703.2)	(619.3)
Total properties, plants and equipment, net	1,593.4	1,480.0
Coke and energy plant, machinery and equipment
Components of net properties, plants and equipment
Gross investment, at cost	1,715.3	1,676.3
Less: Accumulated depreciation	(371.7)	(262.4)
Coal logistics plant, machinery and equipment
Components of net properties, plants and equipment
Gross investment, at cost	159.4	83.6
Land and land improvements
Components of net properties, plants and equipment
Gross investment, at cost	136.6	92.1
Mining
Components of net properties, plants and equipment
Gross investment, at cost	148.4	151.5
Total properties, plants and equipment, net	13.1	22.8
Construction-in-progress
Components of net properties, plants and equipment
Gross investment, at cost	106.1	65.6
Other
Components of net properties, plants and equipment
Gross investment, at cost	$ 30.8	$ 30.2