Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Numerator
Net Income	$ 253.5	$ 153.0
Denominator
Weighted Average Common Shares - Basic	202,628,800	204,133,300
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	875,500	604,000.0
Weighted Average Common Shares - Assuming Dilution	203,504,300	204,737,300
Net Income Per Common Share
Basic	$ 1.25	$ 0.75
Assuming Dilution	$ 1.25	$ 0.75