Share-Based Compensation - Share-Based Compensation Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation Related Costs [Abstract]
Total expense, pre-tax	$ 53.7	$ 57.3	$ 46.4
Tax benefit related to awards	12.5	31.5	14.5
Total expense, net of tax	$ 41.2	$ 25.8	$ 31.9