Summary of Significant Accounting Policies - Earnings Per Common Share (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended						9 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share [Line Items]
Net income for common stock	$ 440	$ 214	$ 277	$ 383	$ 165	$ 311	$ 931	$ 860
Weighted average common shares outstanding - basic	292.9			292.9			292.9	292.5
Add: Incremental shares attributable to effect of potentially dilutive securities	1.7			1.7			1.7	1.7
Adjusted weighted average common shares outstanding - diluted	294.6			294.6			294.6	294.2
Net income for common stock per common share - basic	$ 1.50			$ 1.31			$ 3.18	$ 2.94
Net income for common stock per common share - diluted	$ 1.49			$ 1.30			$ 3.16	$ 2.92