Business Segment Information - Sales and Other Operating Revenue by Product or Service (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Sales and other operating revenue:
Sales and other operating revenue	$ 323.2	$ 292.7	$ 632.9	$ 603.8
Coke sales
Sales and other operating revenue:
Sales and other operating revenue	284.7	258.4	548.8	531.8
Steam and electricity sales
Sales and other operating revenue:
Sales and other operating revenue	10.9	14.4	24.6	28.9
Operating and licensing fees
Sales and other operating revenue:
Sales and other operating revenue	10.5	7.4	21.3	15.1
Coal Logistics
Sales and other operating revenue:
Sales and other operating revenue	14.3	11.0	32.4	23.6
Other
Sales and other operating revenue:
Sales and other operating revenue	$ 2.8	$ 1.5	$ 5.8	$ 4.4