CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interests [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting new hedging standard	$ 0.0				$ 0.0	$ 0.0
Balance at beginning of period at Dec. 31, 2015		$ 1.8	$ 2,266.8	$ (2.0)	(2,963.7)	(213.3)	$ 382.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock		1.3	599.1
Share-based compensation		0.0
Share-based compensation			5.4
Stock options exercised			5.4
Exchangeable notes exchange feature			(21.3)
Purchase of treasury stock				(0.4)
Net income attributable to AK Steel Holding Corporation	(16.8)				(16.8)
Change in accumulated other comprehensive loss	124.6					124.6
Net income attributable to noncontrolling interests	66.0						66.0
Net distributions to noncontrolling interests							(85.1)
Balance at end of period at Dec. 31, 2016	149.8	3.1	2,855.4	(2.4)	(2,980.5)	(88.7)	362.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting new hedging standard	0.0				0.0	0.0
Issuance of common stock		0.0	0.0
Share-based compensation		0.1
Share-based compensation			7.6
Stock options exercised			0.5
Exchangeable notes exchange feature			21.3
Purchase of treasury stock				(3.0)
Net income attributable to AK Steel Holding Corporation	103.5				103.5
Change in accumulated other comprehensive loss	38.5					38.5
Net income attributable to noncontrolling interests	61.4						61.4
Net distributions to noncontrolling interests							(79.1)
Balance at end of period at Dec. 31, 2017	300.6	3.2	2,884.8	(5.4)	(2,877.0)	(50.2)	345.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adopting new hedging standard	0.8				(0.8)	0.8
Issuance of common stock		0.0	0.0
Share-based compensation		0.0
Share-based compensation			9.2
Stock options exercised			0.9
Exchangeable notes exchange feature			0.0
Purchase of treasury stock				(1.0)
Net income attributable to AK Steel Holding Corporation	186.0				186.0
Change in accumulated other comprehensive loss	(50.6)					(50.6)
Net income attributable to noncontrolling interests	58.1						58.1
Net distributions to noncontrolling interests							(73.7)
Balance at end of period at Dec. 31, 2018	$ 429.5	$ 3.2	$ 2,894.9	$ (6.4)	$ (2,691.8)	$ (100.0)	$ 329.6