Earnings Per Share Computation (Detail) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income attributable to common shareholders	$ 128,200,000	$ 129,300,000	$ 133,900,000	$ 138,700,000	$ 113,800,000	$ 130,100,000	$ (47,800,000)	$ 131,900,000	$ 530,100,000	$ 328,000,000	$ 509,200,000
Less: Income allocated to participating securities									10,000,000	6,100,000	5,500,000
Net income available to common shareholders									$ 520,100,000	$ 321,900,000	$ 503,700,000
Weighted average common shares outstanding									83.3	85.8	93.4
Dilutive common share equivalents from share-based compensation plans									1.1	1.5	1.2
Weighted average common and common equivalent shares outstanding, assuming dilution									84.4	87.3	94.6