Summary of Significant Accounting Policies - Earnings Per Common Share (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Earnings Per Share [Abstract]
Net income for common stock	$ 361	$ 192
Weighted average common shares outstanding - basic	292.9	292.9
Add: Incremental shares attributable to effect of potentially dilutive securities	1.2	1.3
Adjusted weighted average common shares outstanding - diluted	294.1	294.2
Net Income for common stock per common share - basic	$ 1.23	$ 0.66
Net Income for common stock per common share - diluted	$ 1.23	$ 0.65