Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator
Net Income	$ 245.1	$ 236.8	$ 475.0	$ 447.4
Denominator (000s)
Weighted Average Common Shares - Basic	227,454,400	236,892,800	228,437,000	238,254,300
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	724,300	426,000	836,800	368,500
Weighted Average Common Shares - Assuming Dilution	228,178,700	237,318,800	229,273,800	238,622,800
Net Income Per Common Share
Basic	$ 1.08	$ 1.00	$ 2.08	$ 1.88
Assuming Dilution	$ 1.07	$ 1.00	$ 2.07	$ 1.87