Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator
Net Income	$ 153.0	$ 161.0
Denominator
Weighted Average Common Shares - Basic	204,133,300	203,306,000.0
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	604,000.0	49,700
Weighted Average Common Shares - Assuming Dilution	204,737,300	203,355,700
Net Income Per Common Share
Basic	$ 0.75	$ 0.79
Assuming Dilution	$ 0.75	$ 0.79