Earnings Per Share Computation (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 116.2	$ 139.8
Less: Income allocated to participating securities	0.6	2.1
Net income available to common shareholders	$ 115.6	$ 137.7
Weighted average common shares outstanding	74.0	74.4
Dilutive common share equivalents from share-based compensation plans	1.2	1.4
Weighted average common and common equivalent shares outstanding, assuming dilution	75.2	75.8