Summary of Significant Accounting Policies - Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Sep. 30, 2015	Sep. 30, 2014
Accounting Policies [Abstract]
Net income for common stock	$ 428	$ 219	$ 370	$ 436	$ 212	$ 361	$ 1,017	$ 1,010
Weighted average common shares outstanding – basic	292.9			292.9			292.9	292.9
Add: Incremental shares attributable to effect of potentially dilutive securities	1.3			1.1			1.3	1.1
Adjusted weighted average common shares outstanding – diluted	294.2			294.0			294.2	294.0
Net Income for common stock per common share - basic (dollars per share)	$ 1.46			$ 1.49			$ 3.47	$ 3.45
Net Income for common stock per common share - diluted (dollars per share)	$ 1.45			$ 1.48			$ 3.46	$ 3.44