Supplementary Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplementary Cash Flow [Line Items]
Net cash flows from operating activities	$ 265.9	$ 364.7	$ 198.8
Cash and cash equivalents	31.0	48.6	38.0	$ 173.2
Supplementary cash flow, noncash investing and financing activities [Abstract]
Capital investments	50.9	33.4	37.3
Interest, net of capitalized interest	137.5	136.3	130.5
Income taxes	(9.5)	(5.5)	0.1
Restricted stock and RSUs [Member]
Supplementary cash flow, noncash investing and financing activities [Abstract]
Issuance of restricted stock and restricted stock units	2.8	4.5	4.6
SunCoke Middletown [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Supplementary Cash Flow [Line Items]
Net cash flows from operating activities	68.6	76.6	$ 77.1
Cash and cash equivalents	$ 0.1	$ 1.1