Earnings Per Share Computation (Detail) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 131,300,000	$ 93,200,000	$ 160,300,000	$ 22,300,000
Less: Income allocated to participating securities	2,200,000	1,600,000	2,600,000	300,000
Net income available to common shareholders	$ 129,100,000	$ 91,600,000	$ 157,700,000	$ 22,000,000
Weighted average common shares outstanding	74.8	78.5	75.7	79.9
Dilutive common share equivalents from share-based compensation plans	1.5	1.5	1.5	1.3
Weighted average common and common equivalent shares outstanding, assuming dilution	76.3	80.0	77.2	81.2