Supplementary Financial Statement Information (Details3) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of products sold (exclusive of item shown separately below)	$ 1,463.7	$ 1,276.9
Income tax expense (benefit)	(4.9)	0.0
Net income	44.8	100.6
Net income attributable to AK Steel Holding Corporation	$ 28.7	$ 84.4
Basic	$ 0.09	$ 0.27
Diluted	$ 0.09	$ 0.26
Reclassification of losses (gains) to net income (loss)	$ (7.6)	$ (0.6)
Comprehensive income	38.1	80.5
Comprehensive income attributable to AK Steel Holding Corporation	22.0	64.3
Inventory	1,334.0			$ 1,385.0
Other non-current assets	412.0			417.5
Other non-current liabilities	150.9			168.9
Accumulated deficit	(2,849.1)			(2,877.0)
Accumulated other comprehensive loss	(56.1)			(50.2)
Deferred income taxes	(6.8)	1.5
Changes in working capital	0.2	(92.5)
Other operating items, net	(13.3)	(8.0)
Scenario, Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of products sold (exclusive of item shown separately below)	1,478.9	1,312.2	[1]
Income tax expense (benefit)	(8.2)	(13.4)
Net income	32.9	78.7
Net income attributable to AK Steel Holding Corporation	$ 16.8	$ 62.5
Basic	$ 0.05	$ 0.20
Diluted	$ 0.05	$ 0.19
Reclassification of losses (gains) to net income (loss)	$ (5.9)	$ (3.2)
Comprehensive income	27.9	56.0
Comprehensive income attributable to AK Steel Holding Corporation	11.8	39.8
Inventory	1,083.3			1,147.8
Other non-current assets	479.1			476.0
Other non-current liabilities	148.9			161.6
Accumulated deficit	(3,042.6)			(3,058.6)
Accumulated other comprehensive loss	(44.2)			(40.0)
Deferred income taxes	(10.1)	(11.9)
Changes in working capital	13.7	(54.6)
Other operating items, net	(11.6)	(10.6)
Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of products sold (exclusive of item shown separately below)	(15.2)	(35.3)
Income tax expense (benefit)	3.3	13.4
Net income	11.9	21.9
Net income attributable to AK Steel Holding Corporation	$ 11.9	$ 21.9
Basic	$ 0.04	$ 0.07
Diluted	$ 0.04	$ 0.07
Reclassification of losses (gains) to net income (loss)	$ (1.7)	$ 2.6
Comprehensive income	10.2	24.5
Comprehensive income attributable to AK Steel Holding Corporation	10.2	24.5
Inventory	250.7			237.2
Other non-current assets	(67.1)			(58.5)
Other non-current liabilities	2.0			7.3
Accumulated deficit	193.5			181.6
Accumulated other comprehensive loss	(11.9)			$ (10.2)
Deferred income taxes	3.3	13.4
Changes in working capital	(13.5)	(37.9)
Other operating items, net	$ (1.7)	$ 2.6

[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.