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Earnings Per Share (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Earnings Per Share [Abstract]                      
Net income attributable to AK Steel Holding Corporation $ 33.5 $ 67.2 $ 56.6 $ 28.7 $ (80.4) $ 22.3 $ 77.2 $ 84.4 $ 186.0 $ 103.5 $ (16.8)
Less: distributed earnings to common stockholders and holders of certain stock compensation awards                 0.0 0.0 0.0
Undistributed earnings (loss)                 186.0 103.5 (16.8)
Common stockholders earnings - basic [Abstract]                      
Distributed earnings to common stockholders                 0.0 0.0 0.0
Undistributed earnings (loss) to common stockholders - basic                 185.7 103.5 (16.7)
Common stockholders earnings (loss) - basic                 185.7 103.5 (16.7)
Common stockholders earnings - diluted [Abstract]                      
Distributed earnings to common stockholders                 0.0 0.0 0.0
Undistributed earnings to common stockholders - diluted                 185.7 103.5 (16.7)
Common stockholders earnings - diluted                 $ 185.7 $ 103.5 $ (16.7)
Common shares outstanding (weighted-average shares in millions) [Abstract]                      
Common shares outstanding for basic earnings per share (in shares)                 314.8 314.3 230.0
Effect of exchangeable debt                 0.0 4.5 0.0
Effect of dilutive stock-based compensation (in shares)                 0.8 0.9 0.0
Common shares outstanding for diluted earnings per share (in shares)                 315.6 319.7 230.0
Basic earnings per share [Abstract]                      
Distributed earnings - basic (in dollars per share)                 $ 0 $ 0 $ 0
Undistributed earnings (loss) - basic (in dollars per share)                 0.59 0.33 (0.07)
Basic earnings (loss) per share (in dollars per share)         $ (0.26) $ 0.07 $ 0.25 $ 0.27 0.59 0.33 (0.07)
Diluted earnings per share [Abstract]                      
Distributed earnings - diluted (in dollars per share)                 0 0 0
Undistributed earnings (loss) - diluted (in dollars per share)                 0.59 0.32 (0.07)
Diluted earnings (loss) per share (in dollars per share)         $ (0.26) $ 0.07 $ 0.24 $ 0.26 $ 0.59 $ 0.32 $ (0.07)
Potentially issuable common shares excluded from earnings per share calculation due to anti-dilutive effect (in millions of shares)                 2.4 1.4 3.0