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Consolidated Statement of Shareholders Equity (Parenthetical) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Sep. 30, 2017
 [1]
Jun. 30, 2017
Mar. 31, 2017
Sep. 30, 2016
 [1]
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Common stock cash dividends (in dollars per share) $ 1.95 $ 0.94 $ 0.94 $ 1.83 $ 0.89 $ 0.89 $ 3.83 $ 3.61 $ 3.44
Stock option exercises and other, tax benefits             $ 0.0 $ (0.6) $ (44.8)
[1] Includes a $0.94 and $0.89 per share dividend declared and paid in third quarter of 2017 and 2016, respectively, and a $1.01 and $0.94 per share dividend declared in the third quarter and paid in fourth quarter of 2017 and 2016, respectively.