Summary of Information Relating to Stock Option Exercises (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Proceeds from stock option exercises	$ 48.8	$ 71.8	$ 83.9
Aggregate intrinsic value	88.3	71.3	72.9
Tax benefit realized upon exercise	$ 31.2	$ 24.3	$ 26.0