Earnings Per Share Computation (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 178.1	$ 116.2
Less: Income allocated to participating securities	0.9	0.6
Net income available to common shareholders	$ 177.2	$ 115.6
Weighted average common shares outstanding	73.1	74.0
Dilutive common share equivalents from share-based compensation plans	1.6	1.2
Weighted average common and common equivalent shares outstanding, assuming dilution	74.7	75.2