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Share-Based Compensation - Compensation Expense and Unrecognized Compensation Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Stock options
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Unrecognized Compensation Cost	$ 1.1		$ 1.1
Recognition Period			1 year 10 months 24 days
Stock options \| Selling, General and Administrative Expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Compensation expense	0.3	$ 0.1	$ 0.5	$ 0.3
RSUs
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Unrecognized Compensation Cost	1.2		$ 1.2
Recognition Period			1 year 3 months 18 days
RSUs \| Selling, General and Administrative Expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Compensation expense	0.3	0.1	$ 0.4	0.2
PSUs
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Unrecognized Compensation Cost	4.5		$ 4.5
Recognition Period			2 years
PSUs \| Selling, General and Administrative Expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Compensation expense	0.5	0.5	$ 1.0	0.9
Total equity awards \| Selling, General and Administrative Expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Compensation expense	1.1	0.7	1.9	1.4
Cash RSUs
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Unrecognized Compensation Cost	1.4		$ 1.4
Recognition Period			1 year 8 months 12 days
Cash RSUs \| Selling, General and Administrative Expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Compensation expense	0.3	0.5	$ 0.7	0.6
Cash incentive award \| Suncoke LTCIP
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Unrecognized Compensation Cost	1.2		$ 1.2
Recognition Period			1 year 10 months 24 days
Cash incentive award \| Selling, General and Administrative Expenses \| Suncoke LTCIP
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Compensation expense	0.3	0.3	$ 0.4	0.5
Total liability awards \| Selling, General and Administrative Expenses
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Compensation expense	$ 0.6	$ 0.8	$ 1.1	$ 1.1