Supplemental Condensed Consolidating Financial Information - Consolidating Balance Sheet (Details) - USD ($) $ / shares in Units, $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets
Cash and cash equivalents	$ 102.2		$ 145.7
Receivables	98.9		75.4
Inventories	175.7		110.4
Income tax receivable	3.2		0.7
Other current assets	4.9		2.8
Advances to affiliate	0.0		0.0
Total current assets	384.9		335.0
Notes receivable from affiliate	0.0		0.0
Properties, plants and equipment, net	1,454.8		1,471.1
Goodwill	76.9		76.9
Other intangible assets, net	151.4		156.8
Deferred income taxes	0.0		0.0
Deferred charges and other assets	14.4		5.5
Total assets	2,082.4		2,045.3
Liabilities and Equity
Advances from affiliate	0.0		0.0
Accounts payable	137.2		115.0
Accrued liabilities	49.9		45.6
Deferred revenue	13.5		3.0
Current portion of long-term debt and financing obligation	5.1		3.9
Interest payable	3.9		3.6
Income taxes payable	0.0		0.0
Total current liabilities	209.6		171.1
Long-term debt and financing obligation	828.0		834.5
Payable to affiliate	0.0		0.0
Accrual for black lung benefits	46.4		44.9
Retirement benefit liabilities	24.1		25.2
Deferred income taxes	212.8		254.7
Asset retirement obligations	13.4		14.6
Other deferred credits and liabilities	25.3		17.6
Total liabilities	1,359.6		1,362.6
Equity
Preferred stock, $0.01 par value. Authorized 50,000,000 shares; no issued shares at both June 30, 2019 and December 31, 2018	0.0		0.0
Common stock, $0.01 par value. Authorized 300,000,000 shares; issued 98,036,174 and 72,233,750 shares at June 30, 2019 and December 31, 2018, respectively	1.0		0.7
Treasury stock, 7,477,657 shares at both June 30, 2019 and December 31, 2018	(140.7)		(140.7)
Additional paid-in capital	709.7		488.8
Accumulated other comprehensive loss	(13.0)		(13.1)
Retained earnings	139.5		127.4
Equity investment eliminations	0.0		0.0
Total SunCoke Energy, Inc. stockholders’ equity	696.5		463.1
Noncontrolling interests	26.3		219.6
Total equity	722.8	$ 687.0	682.7	$ 672.4	$ 659.5	$ 659.6
Total liabilities and equity	$ 2,082.4		$ 2,045.3
Preferred stock, par value (in dollars per share)	$ 0.01		$ 0.01
Preferred stock, shares authorized (in shares)	50,000,000		50,000,000
Preferred stock, shares issued (in shares)	0		0
Common stock, par value (in dollars per share)	$ 0.01		$ 0.01
Common stock, shares authorized (in shares)	300,000,000		300,000,000
Common stock, shares, issued (in shares)	98,036,174		72,233,750
Treasury stock, shares (in shares)	7,477,657		7,477,657
Reportable Legal Entities | Issuer
Assets
Cash and cash equivalents	$ 0.3		$ 0.0
Receivables	0.0		0.0
Inventories	0.0		0.0
Income tax receivable	0.0		0.0
Other current assets	0.0		0.0
Advances to affiliate	0.0		0.0
Total current assets	0.3		0.0
Notes receivable from affiliate	0.0		0.0
Properties, plants and equipment, net	0.0		0.0
Goodwill	0.0		0.0
Other intangible assets, net	0.0		0.0
Deferred income taxes	8.1		7.0
Deferred charges and other assets	0.0		0.0
Total assets	8.4		7.0
Liabilities and Equity
Advances from affiliate	130.3		167.3
Accounts payable	0.0		0.0
Accrued liabilities	1.2		1.8
Deferred revenue	0.0		0.0
Current portion of long-term debt and financing obligation	2.3		1.1
Interest payable	0.4		0.4
Income taxes payable	0.7		1.9
Total current liabilities	134.9		172.5
Long-term debt and financing obligation	39.8		41.2
Payable to affiliate	0.0		0.0
Accrual for black lung benefits	0.0		0.0
Retirement benefit liabilities	0.0		0.0
Deferred income taxes	0.0		0.0
Asset retirement obligations	0.0		0.0
Other deferred credits and liabilities	3.8		3.5
Total liabilities	178.5		217.2
Equity
Preferred stock, $0.01 par value. Authorized 50,000,000 shares; no issued shares at both June 30, 2019 and December 31, 2018	0.0		0.0
Common stock, $0.01 par value. Authorized 300,000,000 shares; issued 98,036,174 and 72,233,750 shares at June 30, 2019 and December 31, 2018, respectively	1.0		0.7
Treasury stock, 7,477,657 shares at both June 30, 2019 and December 31, 2018	(140.7)		(140.7)
Additional paid-in capital	709.7		488.9
Accumulated other comprehensive loss	(13.0)		(13.1)
Retained earnings	139.6		127.5
Equity investment eliminations	(866.7)		(673.5)
Total SunCoke Energy, Inc. stockholders’ equity	(170.1)		(210.2)
Noncontrolling interests	0.0		0.0
Total equity	(170.1)		(210.2)
Total liabilities and equity	8.4		7.0
Reportable Legal Entities | Guarantor Subsidiaries
Assets
Cash and cash equivalents	89.3		123.2
Receivables	13.6		13.3
Inventories	13.8		10.6
Income tax receivable	0.0		0.0
Other current assets	4.4		1.8
Advances to affiliate	276.1		281.1
Total current assets	397.2		430.0
Notes receivable from affiliate	0.0		0.0
Properties, plants and equipment, net	52.4		54.3
Goodwill	3.4		3.4
Other intangible assets, net	0.7		1.1
Deferred income taxes	0.0		0.0
Deferred charges and other assets	5.1		5.1
Total assets	458.8		493.9
Liabilities and Equity
Advances from affiliate	0.0		0.0
Accounts payable	18.1		14.7
Accrued liabilities	13.0		13.7
Deferred revenue	0.0		0.0
Current portion of long-term debt and financing obligation	0.0		0.0
Interest payable	0.0		0.0
Income taxes payable	96.3		93.5
Total current liabilities	127.4		121.9
Long-term debt and financing obligation	0.0		0.0
Payable to affiliate	200.0		200.0
Accrual for black lung benefits	11.2		10.9
Retirement benefit liabilities	11.6		12.2
Deferred income taxes	211.8		194.9
Asset retirement obligations	0.0		0.0
Other deferred credits and liabilities	8.0		6.6
Total liabilities	570.0		546.5
Equity
Preferred stock, $0.01 par value. Authorized 50,000,000 shares; no issued shares at both June 30, 2019 and December 31, 2018	0.0		0.0
Common stock, $0.01 par value. Authorized 300,000,000 shares; issued 98,036,174 and 72,233,750 shares at June 30, 2019 and December 31, 2018, respectively	0.0		0.0
Treasury stock, 7,477,657 shares at both June 30, 2019 and December 31, 2018	0.0		0.0
Additional paid-in capital	201.8		61.0
Accumulated other comprehensive loss	(2.1)		(2.0)
Retained earnings	545.7		526.1
Equity investment eliminations	(856.6)		(637.7)
Total SunCoke Energy, Inc. stockholders’ equity	(111.2)		(52.6)
Noncontrolling interests	0.0		0.0
Total equity	(111.2)		(52.6)
Total liabilities and equity	458.8		493.9
Reportable Legal Entities | Non- Guarantor Subsidiaries
Assets
Cash and cash equivalents	12.6		22.5
Receivables	85.3		62.1
Inventories	161.9		99.8
Income tax receivable	100.2		96.1
Other current assets	0.5		1.0
Advances to affiliate	0.0		0.0
Total current assets	360.5		281.5
Notes receivable from affiliate	200.0		200.0
Properties, plants and equipment, net	1,402.4		1,416.8
Goodwill	73.5		73.5
Other intangible assets, net	150.7		155.7
Deferred income taxes	0.0		0.0
Deferred charges and other assets	9.3		0.4
Total assets	2,196.4		2,127.9
Liabilities and Equity
Advances from affiliate	145.8		113.8
Accounts payable	119.1		100.3
Accrued liabilities	35.7		30.1
Deferred revenue	13.5		3.0
Current portion of long-term debt and financing obligation	2.8		2.8
Interest payable	3.5		3.2
Income taxes payable	0.0		0.0
Total current liabilities	320.4		253.2
Long-term debt and financing obligation	788.2		793.3
Payable to affiliate	0.0		0.0
Accrual for black lung benefits	35.2		34.0
Retirement benefit liabilities	12.5		13.0
Deferred income taxes	9.1		66.8
Asset retirement obligations	13.4		14.6
Other deferred credits and liabilities	13.5		7.5
Total liabilities	1,192.3		1,182.4
Equity
Preferred stock, $0.01 par value. Authorized 50,000,000 shares; no issued shares at both June 30, 2019 and December 31, 2018	0.0		0.0
Common stock, $0.01 par value. Authorized 300,000,000 shares; issued 98,036,174 and 72,233,750 shares at June 30, 2019 and December 31, 2018, respectively	0.0		0.0
Treasury stock, 7,477,657 shares at both June 30, 2019 and December 31, 2018	0.0		0.0
Additional paid-in capital	804.8		612.8
Accumulated other comprehensive loss	(10.9)		(11.1)
Retained earnings	183.9		124.2
Equity investment eliminations	0.0		0.0
Total SunCoke Energy, Inc. stockholders’ equity	977.8		725.9
Noncontrolling interests	26.3		219.6
Total equity	1,004.1		945.5
Total liabilities and equity	2,196.4		2,127.9
Combining and Consolidating Adjustments
Assets
Cash and cash equivalents	0.0		0.0
Receivables	0.0		0.0
Inventories	0.0		0.0
Income tax receivable	(97.0)		(95.4)
Other current assets	0.0		0.0
Advances to affiliate	(276.1)		(281.1)
Total current assets	(373.1)		(376.5)
Notes receivable from affiliate	(200.0)		(200.0)
Properties, plants and equipment, net	0.0		0.0
Goodwill	0.0		0.0
Other intangible assets, net	0.0		0.0
Deferred income taxes	(8.1)		(7.0)
Deferred charges and other assets	0.0		0.0
Total assets	(581.2)		(583.5)
Liabilities and Equity
Advances from affiliate	(276.1)		(281.1)
Accounts payable	0.0		0.0
Accrued liabilities	0.0		0.0
Deferred revenue	0.0		0.0
Current portion of long-term debt and financing obligation	0.0		0.0
Interest payable	0.0		0.0
Income taxes payable	(97.0)		(95.4)
Total current liabilities	(373.1)		(376.5)
Long-term debt and financing obligation	0.0		0.0
Payable to affiliate	(200.0)		(200.0)
Accrual for black lung benefits	0.0		0.0
Retirement benefit liabilities	0.0		0.0
Deferred income taxes	(8.1)		(7.0)
Asset retirement obligations	0.0		0.0
Other deferred credits and liabilities	0.0		0.0
Total liabilities	(581.2)		(583.5)
Equity
Preferred stock, $0.01 par value. Authorized 50,000,000 shares; no issued shares at both June 30, 2019 and December 31, 2018	0.0		0.0
Common stock, $0.01 par value. Authorized 300,000,000 shares; issued 98,036,174 and 72,233,750 shares at June 30, 2019 and December 31, 2018, respectively	0.0		0.0
Treasury stock, 7,477,657 shares at both June 30, 2019 and December 31, 2018	0.0		0.0
Additional paid-in capital	(1,006.6)		(673.9)
Accumulated other comprehensive loss	13.0		13.1
Retained earnings	(729.7)		(650.4)
Equity investment eliminations	1,723.3		1,311.2
Total SunCoke Energy, Inc. stockholders’ equity	0.0		0.0
Noncontrolling interests	0.0		0.0
Total equity	0.0		0.0
Total liabilities and equity	$ (581.2)		$ (583.5)