Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Numerator
Net Income	$ 182.9	$ 265.5	$ 335.9	$ 426.5
Denominator
Weighted Average Common Shares - Basic	204,504,500	203,624,300	204,323,100	203,466,900
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	769,300	46,800	686,700	48,300
Weighted Average Common Shares - Assuming Dilution	205,273,800	203,671,100	205,009,800	203,515,200
Net Income Per Common Share
Basic	$ 0.89	$ 1.30	$ 1.64	$ 2.10
Assuming Dilution	$ 0.89	$ 1.30	$ 1.64	$ 2.10