Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator
Net Income	$ 285.5	$ 245.1	$ 559.0	$ 475.0
Denominator (000s)
Weighted Average Common Shares - Basic	220,776,700	227,454,400	221,335,700	228,437,000
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	286,000	724,300	484,500	836,800
Weighted Average Common Shares - Assuming Dilution	221,062,700	228,178,700	221,820,200	229,273,800
Net Income Per Common Share
Basic	$ 1.29	$ 1.08	$ 2.53	$ 2.08
Assuming Dilution	$ 1.29	$ 1.07	$ 2.52	$ 2.07