Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator
Net Income	$ 328.6	$ 231.1	$ 664.5	$ 657.6
Denominator
Weighted Average Common Shares - Basic	204,645,800	203,761,300	204,429,600	203,564,700
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	496,500	117,800	623,300	71,400
Weighted Average Common Shares - Assuming Dilution	205,142,300	203,879,100	205,052,900	203,636,100
Net Income Per Common Share
Basic	$ 1.61	$ 1.13	$ 3.25	$ 3.23
Assuming Dilution	$ 1.60	$ 1.13	$ 3.24	$ 3.23