Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income attributable to AK Steel Holding Corporation	$ 2.8	$ 67.2	$ 65.1	$ 152.5
Common shares outstanding (weighted-average shares in millions):
Common shares outstanding for basic earnings per share	315.8	314.9	315.8	314.8
Effect of exchangeable debt (in shares)	0.0	0.0	0.0	0.0
Effect of dilutive stock-based compensation (in shares)	1.0	1.1	0.7	1.0
Common shares outstanding for diluted earnings per share	316.8	316.0	316.5	315.8
Basic earnings per share (in dollars per share)	$ 0.01	$ 0.21	$ 0.21	$ 0.48
Diluted earnings per share (in dollars per share)	$ 0.01	$ 0.21	$ 0.21	$ 0.48
Potentially issuable common shares (in millions) excluded from earnings per share calculation due to anti-dilutive effect (in shares)	3.7	2.5	3.7	2.3