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, 2014	Mar. 31, 2014	Jun. 30, 2013	Mar. 31, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share [Abstract]
Net income for common stock	$ 212	$ 361	$ 172	$ 192	$ 574	$ 364
Weighted average common shares outstanding - basic	292.9		292.9		292.9	292.9
Add: Incremental shares attributable to effect of potentially dilutive securities	1.1		1.4		1.1	1.4
Adjusted weighted average common shares outstanding - diluted	294.0		294.3		294.0	294.3
Net income for common stock per common share - basic	$ 0.73		$ 0.59		$ 1.96	$ 1.24
Net income for common stock per common share - diluted	$ 0.72		$ 0.59		$ 1.95	$ 1.24