Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income (loss) attributable to AK Steel Holding Corporation	$ 28.7	$ 84.4
Less: distributed earnings to common stockholders and holders of certain stock compensation awards	0.0	0.0
Undistributed earnings	28.7	84.4
Common stockholders earnings—basic and diluted
Distributed earnings - basic and diluted	0.0	0.0
Undistributed earnings to common stockholders - basic and diluted	28.6	84.3
Common stockholders earnings—basic and diluted	28.6	84.3
Common stockholders earnings – diluted:
Distributed earnings to common stockholders - diluted	0.0	0.0
Undistributed earnings to common stockholders - diluted	28.6	84.3
Common stockholders earnings (loss) – diluted	$ 28.6	$ 84.3
Common shares outstanding (weighted-average shares in millions):
Common shares outstanding for basic earnings per share	314.7	314.1
Effect of exchangeable debt (in shares)	0.5	10.3
Effect of dilutive stock-based compensation (in shares)	0.8	1.2
Common shares outstanding for diluted earnings per share	316.0	325.6
Basic earnings per share:
Distributed earnings - basic (in dollars per share)	$ 0	$ 0
Undistributed earnings - basic (in dollars per share)	0.09	0.27
Basic earnings per share (in dollars per share)	0.09	0.27
Diluted earnings per share:
Distributed earnings - diluted (in dollars per share)	0	0
Undistributed earnings - diluted (in dollars per share)	0.09	0.26
Diluted earnings per share (in dollars per share)	$ 0.09	$ 0.26
Potentially issuable common shares (in millions) excluded from earnings per share calculation due to anti-dilutive effect (in shares)	2.2	1.1