Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator
Net Income	$ 236.8	$ 224.3	$ 447.4	$ 437.2
Denominator (000s)
Weighted Average Common Shares - Basic	236,892,800	248,318,600	238,254,300	249,885,000
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	426,000	873,600	368,500	820,500
Weighted Average Common Shares - Assuming Dilution	237,318,800	249,192,200	238,622,800	250,705,500
Net Income Per Common Share
Basic	$ 1.00	$ 0.90	$ 1.88	$ 1.75
Assuming Dilution	$ 1.00	$ 0.90	$ 1.87	$ 1.74