Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Net income (loss) attributable to AK Steel Holding Corporation	$ 6.7	$ (7.2)	$ (363.6)	$ (110.4)
Less: distributed earnings to common stockholders and holders of certain stock compensation awards	0.0	0.0	0.0	0.0
Undistributed Earnings Attributable To Parent	6.7	(7.2)	(363.6)	(110.4)
Common stockholders earnings - basic [Abstract]
Distributed earnings to common stockholders - basic	0.0	0.0	0.0	0.0
Undistributed earnings (loss) to common stockholders - basic	6.6	(7.2)	(362.4)	(110.0)
Common stockholders earnings (loss) - basic	6.6	(7.2)	(362.4)	(110.0)
Common stockholders earnings - diluted [Abstract]
Distributed earnings to common stockholders - diluted	0.0	0.0	0.0	0.0
Undistributed earnings (loss) to common stockholders - diluted	6.6	(7.2)	(362.4)	(110.0)
Common stockholders earnings (loss) - diluted	$ 6.6	$ (7.2)	$ (362.4)	$ (110.0)
Common shares outstanding (weighted-average shares in millions) [Abstract]
Common shares outstanding for basic earnings per share (in shares)	177.2	142.8	177.1	138.4
Effect of exchangeable debt (in shares)	0.0	0.0	0.0	0.0
Effect of dilutive stock-based compensation (in shares)	0.3	0.0	0.0	0.0
Common shares outstanding for diluted earnings per share (in shares)	177.5	142.8	177.1	138.4
Basic earnings per share [Abstract]
Distributed earnings - basic (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.00
Undistributed earnings (loss) - basic (in dollars per share)	0.04	(0.05)	(2.05)	(0.79)
Basic earnings (loss) per share (in dollars per share)	0.04	(0.05)	(2.05)	(0.79)
Diluted earnings per share [Abstract]
Distributed earnings - diluted (in dollars per share)	0.00	0.00	0.00	0.00
Undistributed earnings (loss) - diluted (in dollars per share)	0.04	(0.05)	(2.05)	(0.79)
Diluted earnings (loss) per share (in dollars per share)	$ 0.04	$ (0.05)	$ (2.05)	$ (0.79)
Potentially issuable common shares (in millions) excluded from earnings per share calculation due to anti-dilutive effect (in shares)	3.4	13.3	3.0	10.6

The total amount of dividends declared in the period for each class of stock and the contractual amount of dividends (or interest on participating income bonds) that must be paid for the period (for example, unpaid cumulative dividends). Dividends declared in the current period do not include dividends declared in respect of prior-period unpaid cumulative dividends. Preferred dividends that are cumulative only if earned are deducted only to the extent that they are earned. Includes the adjustments resulting from the assumption that dilutive securities were converted, options or warrants were exercised, or that other shares were issued upon the satisfaction of certain conditions.

No definition available.