Earnings Per Share Computation (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 139.7	$ 159.2	$ 317.8	$ 275.4
Less: Income allocated to participating securities	0.7	0.8	1.7	1.4
Net income available to common shareholders	$ 139.0	$ 158.4	$ 316.1	$ 274.0
Weighted average common shares outstanding	73.1	74.0	73.1	74.0
Dilutive common share equivalents from share-based compensation plans	1.7	1.2	1.6	1.2
Weighted average common and common equivalent shares outstanding, assuming dilution	74.8	75.2	74.7	75.2