Income Per Common Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 78.3	$ 69.8	$ 85.6	$ 72.6
Participating Securities, Distributed and Undistributed Earnings (Loss), Basic	0.0	(1.3)	(0.1)	(1.3)
Income (Loss) from Continuing Operations, Basic	78.3	68.5	85.5	71.3
Net Income (Loss) Available to Common Stockholders, Diluted	$ 78.3	$ 69.8	$ 85.6	$ 72.6
Earnings Per Share, Basic and Diluted [Abstract]
Basic	16.1	16.5	16.2	16.5
Plus dilutive effect of stock awards	0.2	0.2	0.2	0.2
Plus dilutive effect of participating securities	0.0	0.3	0.1	0.3
Diluted	16.3	17.0	16.5	17.0
Earnings Per Share, Diluted [Abstract]
Basic	$ 4.86	$ 4.16	$ 5.27	$ 4.32
Diluted	$ 4.81	$ 4.11	$ 5.18	$ 4.27