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, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2016	Sep. 30, 2015
Accounting Policies [Abstract]
Net income	$ 497	$ 232	$ 310	$ 428	$ 219	$ 370	$ 1,039	$ 1,017
Weighted average common shares outstanding – basic	304.5			292.9			299.1	292.9
Add: Incremental shares attributable to effect of potentially dilutive securities	1.4			1.3			1.4	1.3
Adjusted weighted average common shares outstanding – diluted	305.9			294.2			300.5	294.2
Net income per common share-basic (dollars per share)	$ 1.63			$ 1.46			$ 3.47	$ 3.47
Net income per common share-diluted (dollars per share)	$ 1.62			$ 1.45			$ 3.46	$ 3.46