Earnings Per Share Computation (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net (loss) income	$ (86.0)	$ 131.3	$ (0.9)	$ 160.3
Less: Income allocated to participating securities	0.0	2.2	0.0	2.6
Net (loss) income available to common shareholders	$ (86.0)	$ 129.1	$ (0.9)	$ 157.7
Weighted average common shares outstanding	74.1	74.8	74.2	75.7
Dilutive common share equivalents from share-based compensation plans	0.0	1.5	0.0	1.5
Weighted average common and common equivalent shares outstanding, assuming dilution	74.1	76.3	74.2	77.2