Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net income attributable to AK Steel Holding Corporation	$ 61.2	$ 17.3	$ 123.7	$ 3.7
Less: distributed earnings to common stockholders and holders of certain stock compensation awards	0.0	0.0	0.0	0.0
Undistributed earnings	61.2	17.3	123.7	3.7
Common stockholders earnings - basic [Abstract]
Distributed earnings to common stockholders - basic	0.0	0.0	0.0	0.0
Undistributed earnings to common stockholders - basic	61.1	17.2	123.5	3.7
Common stockholders earnings - basic	61.1	17.2	123.5	3.7
Common stockholders earnings - diluted [Abstract]
Distributed earnings to common stockholders - diluted	0.0	0.0	0.0	0.0
Undistributed earnings to common stockholders - diluted	61.1	17.2	123.5	3.7
Common stockholders earnings - diluted	$ 61.1	$ 17.2	$ 123.5	$ 3.7
Common shares outstanding (weighted-average shares in millions) [Abstract]
Common shares outstanding for basic earnings per share	314.3	216.4	314.2	197.0
Effect of exchangeable debt (in shares)	3.9	0.0	7.6	0.0
Effect of dilutive stock-based compensation (in shares)	0.8	0.8	1.0	0.6
Common shares outstanding for diluted earnings per share	319.0	217.2	322.8	197.6
Basic earnings per share [Abstract]
Distributed earnings - basic (in dollars per share)	$ 0	$ 0	$ 0	$ 0
Undistributed earnings - basic (in dollars per share)	0.19	0.08	0.39	0.02
Basic earnings per share (in dollars per share)	0.19	0.08	0.39	0.02
Diluted earnings per share [Abstract]
Distributed earnings - diluted (in dollars per share)	0	0	0	0
Undistributed earnings - diluted (in dollars per share)	0.19	0.08	0.38	0.02
Diluted earnings per share (in dollars per share)	$ 0.19	$ 0.08	$ 0.38	$ 0.02
Potentially issuable common shares (in millions) excluded from earnings per share calculation due to anti-dilutive effect (in shares)	1.5	2.7	1.1	3.3