Summary of Significant Accounting Policies - Earnings Per Common Share (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share [Line Items]
Net income for common stock	$ 1,138	$ 1,051	$ 992
Weighted average common shares outstanding - Basic	292.9	292.6	284.3
Add: Incremental shares attributable to effect of potentially dilutive securities	1.6	1.8	1.6
Adjusted weighted average common shares outstanding - Diluted	294.5	294.4	285.9
Net income for common stock per common share - basic	$ 3.88	$ 3.59	$ 3.49
Net income for common stock per common share - diluted	$ 3.86	$ 3.57	$ 3.47