Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 173.9	$ 18.8	$ (31.5)	$ (57.7)	$ 31.5	$ 14.4	$ 1.0	$ 12.6	$ 103.5	$ 59.5	$ 10.3
Other comprehensive income (loss):
Reclassifications of actuarial loss amortization, prior service benefit, curtailment gain and settlement loss to earnings (net of related tax expense of zero, zero and $3.4 million, respectively)									0.2	0.0	5.2
Retirement benefit plans funded status adjustment (net of related tax benefit of $0.3 million, $0.1 million and $0.1 million, respectively)									(0.8)	(0.2)	(0.4)
Currency translation adjustment									(0.5)	1.0	(3.1)
Comprehensive income									102.4	60.3	12.0
Less: Comprehensive (loss) income attributable to noncontrolling interests									(18.9)	45.1	32.3
Comprehensive income (loss) attributable to SunCoke Energy, Inc.									$ 121.3	$ 15.2	$ (20.3)