Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator
Net Income	$ 252.3	$ 236.0	$ 727.3	$ 683.4
Denominator (000s)
Weighted Average Common Shares - Basic	225,288,100	233,752,000	227,375,500	236,744,300
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	741,700	461,500	805,000	399,500
Weighted Average Common Shares - Assuming Dilution	226,029,800	234,213,500	228,180,500	237,143,800
Net Income Per Common Share
Basic	$ 1.12	$ 1.01	$ 3.20	$ 2.89
Assuming Dilution	$ 1.12	$ 1.01	$ 3.19	$ 2.88