Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator
Net Income	$ 231.1	$ 242.0	$ 657.6	$ 804.1
Denominator
Weighted Average Common Shares - Basic	203,761,300	208,097,800	203,564,700	211,131,500
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	117,800	18,200	71,400	65,700
Weighted Average Common Shares - Assuming Dilution	203,879,100	208,116,000.0	203,636,100	211,197,200
Net Income Per Common Share
Basic	$ 1.13	$ 1.16	$ 3.23	$ 3.81
Assuming Dilution	$ 1.13	$ 1.16	$ 3.23	$ 3.81