Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Supplemental cash flow, net cash paid (received) during the period for: [Abstract]
Interest, net of capitalized interest	$ 1.6	$ 3.2
Income taxes paid, net	0.3	0.2
Net Cash Provided by (Used in) Operating Activities	(7.0)	(150.0)
Supplemental cash flow, noncash investing and financing activities [Abstract]
Capital investments	6.9	4.3
Cash and Cash Equivalents, at Carrying Value	191.8	42.3	227.0	42.0
Restricted Stock and Restricted Stock Units [Member]
Supplemental cash flow, noncash investing and financing activities [Abstract]
Issuance of restricted stock and restricted stock units	2.3	4.5
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	23.7	19.2
Supplemental cash flow, noncash investing and financing activities [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 13.6		$ 0