Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income attributable to AK Steel Holding Corporation	$ (53.9)	$ 2.8	$ 66.8	$ (4.5)	$ 33.5	$ 67.2	$ 56.6	$ 28.7	$ 11.2	$ 186.0	$ 103.5
Common shares outstanding (weighted-average shares in millions) [Abstract]
Common shares outstanding for basic earnings per share (in shares)									315.8	314.8	314.3
Effect of exchangeable debt									0.0	0.0	4.5
Effect of dilutive stock-based compensation (in shares)									0.8	0.8	0.9
Common shares outstanding for diluted earnings per share (in shares)									316.6	315.6	319.7
Basic earnings per share (in dollars per share)									$ 0.04	$ 0.59	$ 0.33
Diluted earnings per share (in dollars per share)									$ 0.04	$ 0.59	$ 0.32
Potentially issuable common shares excluded from earnings per share calculation due to anti-dilutive effect (in millions of shares)									3.6	2.4	1.4