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, 2014	Jun. 30, 2014	Mar. 31, 2014	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Sep. 30, 2014	Sep. 30, 2013
Earnings Per Share [Abstract]
Net income for common stock	$ 436	$ 212	$ 361	$ 464	$ 172	$ 192	$ 1,010	$ 828
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	$ 1.49			$ 1.58			$ 3.45	$ 2.83
Net income for common stock per common share - diluted	$ 1.48			$ 1.58			$ 3.44	$ 2.81