Share-Based Compensation (Compensation Expense and Unrecognized Compensation Costs) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Compensation Cost	$ 0.1
Recognition Period	1 year 1 month 6 days
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Compensation Cost	$ 0.5
Recognition Period	1 year 10 months 24 days
Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Compensation Cost	$ 1.5
Recognition Period	1 year 6 months
Restricted Stock Units Settled in Cash
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Compensation Cost	$ 0.6
Recognition Period	1 year 10 months 24 days
Suncoke LTCIP \| Cash Incentive Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Compensation Cost	$ 1.2
Recognition Period	1 year 10 months 24 days
Selling, General and Administrative Expenses \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 0.3	$ 1.1	$ 0.5
Net of tax	0.3	0.9	0.4
Selling, General and Administrative Expenses \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	1.9	1.0	0.4
Net of tax	1.5	0.9	0.3
Selling, General and Administrative Expenses \| Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	1.5	2.2	1.9
Net of tax	1.2	1.8	1.7
Selling, General and Administrative Expenses \| Equity Classified Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	3.7	4.3	2.8
Net of tax	3.0	3.6	2.4
Selling, General and Administrative Expenses \| Restricted Stock Units Settled in Cash
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	0.8	0.9	0.8
Net of tax	0.6	0.7	0.6
Selling, General and Administrative Expenses \| Suncoke LTCIP \| Cash Incentive Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	0.6	0.4	0.9
Net of tax	0.4	0.3	0.7
Selling, General and Administrative Expenses \| Suncoke LTCIP \| Liability Classified Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	1.4	1.3	1.7
Net of tax	$ 1.0	$ 1.0	$ 1.3

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Amount, after tax, of expense for award under share-based payment arrangement.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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