Supplementary Financial Statement Information - LIFO Recast (Details4) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of products sold										$ 5,911.0	$ 5,253.1	$ 5,099.7
Income tax expense (benefit)										(6.2)	(2.2)	(16.9)
Net income										244.1	164.9	49.2
Net income attributable to AK Steel Holding Corporation		$ 33.5	$ 67.2	$ 56.6	$ 28.7	$ (80.4)	$ 22.3	$ 77.2	$ 84.4	$ 186.0	$ 103.5	$ (16.8)
Earnings Per Share, Basic						$ (0.26)	$ 0.07	$ 0.25	$ 0.27	$ 0.59	$ 0.33	$ (0.07)
Earnings Per Share, Diluted						$ (0.26)	$ 0.07	$ 0.24	$ 0.26	$ 0.59	$ 0.32	$ (0.07)
Reclassification of losses (gains) to net income										$ (10.3)	$ (6.1)	$ 39.1
Comprehensive income										193.5	203.4	173.8
Comprehensive income attributable to AK Steel Holding Corporation										135.4	142.0	107.8
Inventory		1,419.9				$ 1,385.0				1,419.9	1,385.0
Other non-current assets		103.9				110.8				103.9	110.8
Other non-current liabilities		134.0				168.9				134.0	168.9
Accumulated deficit		(2,691.8)				(2,877.0)				(2,691.8)	(2,877.0)
Accumulated other comprehensive loss		(100.0)				(50.2)				(100.0)	(50.2)
Deferred income taxes										(8.0)	(9.0)	(4.1)
Changes in working capital										12.7	(116.7)	129.6
Other operating items, net										2.4	20.1	3.0
AdjustmentAsIfUnderLifo [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of products sold										(55.7)
Income tax expense (benefit)										13.2
Net income										42.5
Net income attributable to AK Steel Holding Corporation										$ 42.5
Earnings Per Share, Basic										$ 0.13
Earnings Per Share, Diluted										$ 0.14
Comprehensive income										$ 42.5
Comprehensive income attributable to AK Steel Holding Corporation										42.5
Inventory		291.2								291.2
Other non-current assets		(77.0)								(77.0)
Other non-current liabilities		2.0								2.0
Accumulated deficit		224.1								224.1
Deferred income taxes										13.2
Changes in working capital										(54.0)
Other operating items, net										(1.7)
ChangeFromLifoToAverageCostMethod [Member] | AsIfUnderLifo [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of products sold										5,966.7
Income tax expense (benefit)										(19.4)
Net income										201.6
Net income attributable to AK Steel Holding Corporation										$ 143.5
Earnings Per Share, Basic										$ 0.46
Earnings Per Share, Diluted										$ 0.45
Comprehensive income										$ 151.0
Comprehensive income attributable to AK Steel Holding Corporation										92.9
Inventory		1,128.7								1,128.7
Other non-current assets		180.9								180.9
Other non-current liabilities		132.0								132.0
Accumulated deficit		$ (2,915.9)								(2,915.9)
Deferred income taxes										(21.2)
Changes in working capital										66.7
Other operating items, net										$ 4.1
Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of products sold	[1]										5,365.2	5,070.6
Income tax expense (benefit)											(17.0)	3.2
Net income											67.6	58.2
Net income attributable to AK Steel Holding Corporation											$ 6.2	$ (7.8)
Earnings Per Share, Basic											$ 0.02	$ (0.03)
Earnings Per Share, Diluted											$ 0.02	$ (0.03)
Reclassification of losses (gains) to net income											$ (12.5)	$ 27.2
Comprehensive income											91.1	181.9
Comprehensive income attributable to AK Steel Holding Corporation											29.7	115.9
Inventory						1,147.8					1,147.8
Other non-current assets						169.3					169.3
Other non-current liabilities						161.6					161.6
Accumulated deficit						(3,058.6)					(3,058.6)
Accumulated other comprehensive loss						(40.0)					(40.0)
Deferred income taxes											(15.2)	5.0
Changes in working capital											1.8	112.4
Other operating items, net											5.1	2.1
Restatement Adjustment [Member] | ChangeFromLifoToAverageCostMethod [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of products sold											(112.1)	29.1
Income tax expense (benefit)											14.8	(20.1)
Net income											97.3	(9.0)
Net income attributable to AK Steel Holding Corporation											$ 97.3	$ (9.0)
Earnings Per Share, Basic											$ 0.31	$ (0.04)
Earnings Per Share, Diluted											$ 0.30	$ (0.04)
Reclassification of losses (gains) to net income											$ 6.4	$ 11.9
Comprehensive income											112.3	(8.1)
Comprehensive income attributable to AK Steel Holding Corporation											112.3	(8.1)
Inventory						237.2					237.2
Other non-current assets						(58.5)					(58.5)
Other non-current liabilities						7.3					7.3
Accumulated deficit						181.6					181.6
Accumulated other comprehensive loss						$ (10.2)					(10.2)
Deferred income taxes											6.2	(9.1)
Changes in working capital											(118.5)	17.2
Other operating items, net											$ 15.0	$ 0.9

[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.