Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator
Net Income	$ 265.5	$ 281.2	$ 426.5	$ 562.1
Denominator
Weighted Average Common Shares - Basic	203,624,300	211,068,700	203,466,900	212,672,800
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	46,800	43,900	48,300	88,300
Weighted Average Common Shares - Assuming Dilution	203,671,100	211,112,600	203,515,200	212,761,100
Net Income Per Common Share
Basic	$ 1.30	$ 1.33	$ 2.10	$ 2.64
Assuming Dilution	$ 1.30	$ 1.33	$ 2.10	$ 2.64