Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income (loss) attributable to AK Steel Holding Corporation	$ (5.8)	$ 50.9	$ 117.9	$ 54.6
Less: distributed earnings to common stockholders and holders of certain stock compensation awards	0.0	0.0	0.0	0.0
Undistributed earnings	(5.8)	50.9	117.9	54.6
Common stockholders earnings—basic and diluted
Distributed earnings - basic and diluted	0.0	0.0	0.0	0.0
Undistributed earnings to common stockholders - basic and diluted	(5.8)	50.8	117.7	54.4
Common stockholders earnings—basic and diluted	(5.8)	50.8	117.7	54.4
Common stockholders earnings – diluted:
Distributed earnings to common stockholders - diluted	0.0	0.0	0.0	0.0
Undistributed earnings to common stockholders - diluted	(5.8)	50.8	117.7	54.4
Common stockholders earnings (loss) – diluted	$ (5.8)	$ 50.8	$ 117.7	$ 54.4
Common shares outstanding (weighted-average shares in millions):
Common shares outstanding for basic earnings per share	314.3	237.5	314.2	210.6
Effect of exchangeable debt (in shares)	0.0	0.0	5.9	0.0
Effect of dilutive stock-based compensation (in shares)	0.0	0.9	1.0	0.8
Common shares outstanding for diluted earnings per share	314.3	238.4	321.1	211.4
Basic earnings per share:
Distributed earnings - basic (in dollars per share)	$ 0	$ 0	$ 0	$ 0
Undistributed earnings - basic (in dollars per share)	(0.02)	0.21	0.37	0.26
Basic earnings per share (in dollars per share)	(0.02)	0.21	0.37	0.26
Diluted earnings per share:
Distributed earnings - diluted (in dollars per share)	0	0	0	0
Undistributed earnings - diluted (in dollars per share)	(0.02)	0.21	0.37	0.26
Diluted earnings per share (in dollars per share)	$ (0.02)	$ 0.21	$ 0.37	$ 0.26
Potentially issuable common shares (in millions) excluded from earnings per share calculation due to anti-dilutive effect (in shares)	4.1	1.9	1.1	2.3