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, 2016	Mar. 31, 2016	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015
Accounting Policies [Abstract]
Net income	$ 232	$ 310	$ 219	$ 370	$ 542	$ 589
Weighted average common shares outstanding – basic	299.1		292.9		296.7	292.9
Add: Incremental shares attributable to effect of potentially dilutive securities	1.3		1.1		1.3	1.0
Adjusted weighted average common shares outstanding – diluted	300.4		294.0		298.0	293.9
Net income per common share-basic (dollars per share)	$ 0.78		$ 0.75		$ 1.83	$ 2.01
Net income per common share-diluted (dollars per share)	$ 0.77		$ 0.74		$ 1.82	$ 2.01