Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator
Net Income	$ 281.2	$ 285.5	$ 562.1	$ 559.0
Denominator
Weighted Average Common Shares - Basic	211,068,700	220,776,700	212,672,800	221,335,700
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	43,900	286,000.0	88,300	484,500
Weighted Average Common Shares - Assuming Dilution	211,112,600	221,062,700	212,761,100	221,820,200
Net Income Per Common Share
Basic	$ 1.33	$ 1.29	$ 2.64	$ 2.53
Assuming Dilution	$ 1.33	$ 1.29	$ 2.64	$ 2.52