Supplementary Cash Flow Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Supplemental cash flow, net cash paid (received) during the period for: [Abstract]
Interest, net of capitalized interest	$ 32.8	$ 45.1
Income taxes	(7.5)	0.1
Net Cash Provided by (Used in) Operating Activities	65.6	36.3
Cash and Cash Equivalents, at Carrying Value	44.5	185.7	$ 38.0	$ 173.2
Supplemental cash flow, noncash investing and financing activities [Abstract]
Capital investments	20.0	21.6
Restricted Stock and Restricted Stock Units [Member]
Supplemental cash flow, noncash investing and financing activities [Abstract]
Issuance of restricted stock and restricted stock units	3.7	3.8
SunCoke Middletown [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Supplemental cash flow, net cash paid (received) during the period for: [Abstract]
Net Cash Provided by (Used in) Operating Activities	21.5	$ 20.3
Cash and Cash Equivalents, at Carrying Value	$ 10.6		$ 0.1