Summary of Significant Accounting Policies - Earnings Per Common Share (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013	Mar. 31, 2013	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share [Abstract]
Net income for common stock	$ 172	$ 192	$ 214	$ 277	$ 364	$ 491
Weighted average common shares outstanding - basic	292.9		292.9		292.9	292.9
Add: Incremental shares attributable to effect of potentially dilutive securities	1.4		1.5		1.4	1.5
Adjusted weighted average common shares outstanding - diluted	294.3		294.4		294.3	294.4
Net Income for common stock per common share - basic	$ 0.59		$ 0.73		$ 1.24	$ 1.68
Net Income for common stock per common share - diluted	$ 0.59		$ 0.73		$ 1.24	$ 1.67