Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator
Net Income	$ 273.5	$ 229.9
Denominator (000s)
Weighted Average Common Shares - Basic	221,894,000	229,429,600
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	683,000	949,200
Weighted Average Common Shares - Assuming Dilution	222,577,000	230,378,800
Net Income Per Common Share
Basic	$ 1.23	$ 1.00
Assuming Dilution	$ 1.23	$ 1.00