Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income (loss) attributable to AK Steel Holding Corporation	$ 56.6	$ 77.2	$ 85.3	$ 161.6
Less: distributed earnings to common stockholders and holders of certain stock compensation awards	0.0	0.0	0.0	0.0
Undistributed earnings	56.6	77.2	85.3	161.6
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	56.6	77.1	85.2	161.4
Common stockholders earnings—basic and diluted	56.6	77.1	85.2	161.4
Common stockholders earnings – diluted:
Undistributed earnings to common stockholders - diluted	56.6	77.1	85.2	161.4
Common stockholders earnings (loss) – diluted	$ 56.6	$ 77.1	$ 85.2	$ 161.4
Common shares outstanding (weighted-average shares in millions):
Common shares outstanding for basic earnings per share	314.7	314.3	314.7	314.2
Effect of exchangeable debt (in shares)	0.0	3.9	0.0	7.6
Effect of dilutive stock-based compensation (in shares)	1.0	0.8	1.0	1.0
Common shares outstanding for diluted earnings per share	315.7	319.0	315.7	322.8
Basic earnings per share:
Distributed earnings - basic (in dollars per share)	$ 0	$ 0	$ 0	$ 0
Undistributed earnings - basic (in dollars per share)	0.18	0.25	0.27	0.51
Basic earnings per share (in dollars per share)	0.18	0.25	0.27	0.51
Diluted earnings per share:
Distributed earnings - diluted (in dollars per share)	0	0	0	0
Undistributed earnings - diluted (in dollars per share)	0.18	0.24	0.27	0.50
Diluted earnings per share (in dollars per share)	$ 0.18	$ 0.24	$ 0.27	$ 0.50
Potentially issuable common shares (in millions) excluded from earnings per share calculation due to anti-dilutive effect (in shares)	2.5	1.5	2.3	1.1