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Acquisitions and Divestitures (Schedule of Non-Controlling Interest) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Subsidiary, Sale of Stock [Line Items]      
Net (loss) income attributable to SunCoke Energy, Inc. $ 3.7 $ (152.3) $ 26.2
Increase (decrease) in SunCoke Energy, Inc. equity for the purchase of additional interest in the Partnership   182.5 (1.2)
Changes from net (loss) income attributable to SunCoke Energy, Inc. and transfers to noncontrolling interest   $ 30.2 $ 25.0
Public Unit Purchase Program      
Subsidiary, Sale of Stock [Line Items]      
Units purchased (in shares)     231,171
Acquisition of additional interest in the Partnership     $ 4.2