Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator
Net Income	$ 280.9	$ 273.5
Denominator (000s)
Weighted-Average Common Shares - Basic	214,297,100	221,894,000
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	132,600	683,000
Weighted-Average Common Shares - Assuming Dilution	214,429,700	222,577,000
Net Income Per Common Share
Basic	$ 1.31	$ 1.23
Assuming Dilution	$ 1.31	$ 1.23