Earnings Per Share Computation (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 159.6	$ 96.4	$ 159.2	$ 116.2	$ 136.3	$ (86.0)	$ (54.7)	$ 139.8	$ 531.4	$ 135.4	$ 294.5
Less: Income allocated to participating securities									2.6	1.9	4.8
Net income available to common shareholders									$ 528.8	$ 133.5	$ 289.7
Weighted average common shares outstanding									74.0	74.1	75.6
Dilutive common share equivalents from share-based compensation plans									1.2	1.3	1.5
Weighted average common and common equivalent shares outstanding, assuming dilution									75.2	75.4	77.1