Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Numerator
Net Income	$ 161.0	$ 280.9
Denominator
Weighted Average Common Shares - Basic	203,306,000.0	214,297,100
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	49,700	132,600
Weighted Average Common Shares - Assuming Dilution	203,355,700	214,429,700
Net Income Per Common Share
Basic	$ 0.79	$ 1.31
Assuming Dilution	$ 0.79	$ 1.31