Supplementary Financial Statement Information (Details3) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of products sold (exclusive of item shown separately below)		$ 1,512.7	$ 1,320.0	$ 2,976.4	$ 2,596.9
Income tax expense (benefit)		(0.5)	0.4	(5.4)	0.4
Net income		72.3	92.4	117.1	193.0
Net income attributable to AK Steel Holding Corporation		$ 56.6	$ 77.2	$ 85.3	$ 161.6
Basic		$ 0.18	$ 0.25	$ 0.27	$ 0.51
Diluted		$ 0.18	$ 0.24	$ 0.27	$ 0.50
Recorded in cost of products sold		$ (5.5)	$ (1.3)	$ (13.1)	$ (1.9)
Comprehensive income		61.6	73.3	99.7	153.8
Comprehensive income attributable to AK Steel Holding Corporation		45.9	58.1	67.9	122.4
Inventory		1,311.0		1,311.0		$ 1,385.0
Other non-current assets		405.7		405.7		417.5
Other non-current liabilities		145.6		145.6		168.9
Accumulated deficit		(2,792.5)		(2,792.5)		(2,877.0)
Accumulated other comprehensive loss		(66.8)		(66.8)		(50.2)
Deferred income taxes				(7.2)	4.6
Changes in working capital				28.8	(57.3)
Other operating items, net				(27.7)	0.9
As If Under LIFO [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of products sold (exclusive of item shown separately below)		1,526.2		3,005.1
Income tax expense (benefit)		(3.8)		(12.0)
Net income		62.1		95.0
Net income attributable to AK Steel Holding Corporation		$ 46.4		$ 63.2
Basic		$ 0.15		$ 0.20
Diluted		$ 0.15		$ 0.20
Comprehensive income		$ 51.4		$ 77.6
Comprehensive income attributable to AK Steel Holding Corporation		35.7		45.8
Inventory		1,046.8		1,046.8
Other non-current assets		476.1		476.1
Other non-current liabilities		143.6		143.6
Accumulated deficit		(2,996.2)		(2,996.2)
Deferred income taxes				(13.8)
Changes in working capital				55.8
Other operating items, net				(26.0)
Adjustment As If Under LIFO [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of products sold (exclusive of item shown separately below)		(13.5)		(28.7)
Income tax expense (benefit)		3.3		6.6
Net income		10.2		22.1
Net income attributable to AK Steel Holding Corporation		$ 10.2		$ 22.1
Basic		$ 0.03		$ 0.07
Diluted		$ 0.03		$ 0.07
Comprehensive income		$ 10.2		$ 22.1
Comprehensive income attributable to AK Steel Holding Corporation		10.2		22.1
Inventory		264.2		264.2
Other non-current assets		(70.4)		(70.4)
Other non-current liabilities		2.0		2.0
Accumulated deficit		$ 203.7		203.7
Deferred income taxes				6.6
Changes in working capital				(27.0)
Other operating items, net				$ (1.7)
Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of products sold (exclusive of item shown separately below)	[1]		1,345.1		2,657.3
Income tax expense (benefit)			(8.7)		(22.1)
Net income			76.4		155.1
Net income attributable to AK Steel Holding Corporation			$ 61.2		$ 123.7
Basic			$ 0.19		$ 0.39
Diluted			$ 0.19		$ 0.38
Recorded in cost of products sold			$ (2.1)		$ (5.3)
Comprehensive income			56.5		112.5
Comprehensive income attributable to AK Steel Holding Corporation			41.3		81.1
Inventory						1,147.8
Other non-current assets						476.0
Other non-current liabilities						161.6
Accumulated deficit						(3,058.6)
Accumulated other comprehensive loss						(40.0)
Deferred income taxes					(17.9)
Changes in working capital					6.5
Other operating items, net					(2.5)
Restatement Adjustment [Member] | ChangeFromLIFOToAverageCostMethod [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of products sold (exclusive of item shown separately below)			(25.1)		(60.4)
Income tax expense (benefit)			9.1		22.5
Net income			16.0		37.9
Net income attributable to AK Steel Holding Corporation			$ 16.0		$ 37.9
Basic			$ 0.06		$ 0.12
Diluted			$ 0.05		$ 0.12
Recorded in cost of products sold			$ 0.8		$ 3.4
Comprehensive income			16.8		41.3
Comprehensive income attributable to AK Steel Holding Corporation			$ 16.8		41.3
Inventory						237.2
Other non-current assets						(58.5)
Other non-current liabilities						7.3
Accumulated deficit						181.6
Accumulated other comprehensive loss						$ (10.2)
Deferred income taxes					22.5
Changes in working capital					(63.8)
Other operating items, net					$ 3.4

[1] (a)Cost of products sold as originally reported reflects the change in presentation of pension and OPEB (income) expense further described in Note 1.