Summary of Significant Accounting Policies - Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accounting Policies [Abstract]
Net income	$ 310	$ 370
Weighted average common shares outstanding – basic	293.7	292.9
Add: Incremental shares attributable to effect of potentially dilutive securities	1.1	1.0
Adjusted weighted average common shares outstanding – diluted	294.8	293.9
Net income per common share-basic (dollars per share)	$ 1.05	$ 1.26
Net income per common share-diluted (dollars per share)	$ 1.05	$ 1.26