Earnings Per Share Computation (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 94.1	$ 96.4	$ 411.9	$ 371.8
Less: Income allocated to participating securities	0.5	0.4	2.2	1.8
Net income available to common shareholders	$ 93.6	$ 96.0	$ 409.7	$ 370.0
Weighted average common shares outstanding	73.4	74.1	73.2	74.0
Dilutive common share equivalents from share-based compensation plans	1.7	1.2	1.7	1.2
Weighted average common and common equivalent shares outstanding, assuming dilution	75.1	75.3	74.9	75.2