Supplementary Cash Flow Information (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Supplemental cash flow, net cash paid (received) during the period for: [Abstract]
Interest, net of capitalized interest	$ 101.1	$ 82.7
Income taxes	(0.2)	(2.6)
Net Cash Provided by (Used in) Operating Activities	154.0	287.9
Cash and Cash Equivalents, at Carrying Value	69.3	57.5	$ 173.2	$ 56.6
Supplemental cash flow, noncash investing and financing activities [Abstract]
Capital investments	29.0	42.7
Restricted Stock and Restricted Stock Units [Member]
Supplemental cash flow, noncash investing and financing activities [Abstract]
Issuance of performance shares, restricted stock and restricted stock units	4.3	2.0
Variable Interest Entity, Primary Beneficiary [Member] | SunCoke Middletown [Member]
Supplemental cash flow, net cash paid (received) during the period for: [Abstract]
Net Cash Provided by (Used in) Operating Activities	59.2	68.0
Research And Innovation Center Middletown OH [Member]
Supplemental cash flow, noncash investing and financing activities [Abstract]
Research and Innovation Center capital lease	1.1	$ 21.6
SunCoke Middletown [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Supplemental cash flow, net cash paid (received) during the period for: [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 4.5		$ 5.0