Variable Interest Entities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Variable Interest Entity [Line Items]
Income (loss) before income taxes	$ 39.9	$ 100.6
SunCoke Middletown [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Variable interest entity, ownership percentage (in hundredths)	0.00%
Income (loss) before income taxes	$ 16.2	$ 16.2
Vicksmetal Armco Associates [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Variable interest entity, ownership percentage (in hundredths)	50.00%