Summary of Significant Accounting Policies - Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2015	Jun. 30, 2014
Accounting Policies [Abstract]
Net income for common stock	$ 219	$ 370	$ 212	$ 361	$ 589	$ 574
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		1.1		1.0	1.1
Adjusted weighted average common shares outstanding – diluted	294.0		294.0		293.9	294.0
Net Income for common stock per common share - basic (dollars per share)	$ 0.75		$ 0.73		$ 2.01	$ 1.96
Net Income for common stock per common share - diluted (dollars per share)	$ 0.74		$ 0.72		$ 2.01	$ 1.95