Common Stock - Stock Options Status and Activity (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares Under Option Outstanding Beginning Balance	1,186,655	1,187,404	1,336,579	1,454,003	1,454,003
Share Under Option, Canceled				(20,427)
Shares Under Option, Forfeited	(5,066)		(7,531)
Shares Under Option, Exercised	(36,870)	(749)	(141,644)	(96,997)
Shares Under Option Outstanding Ending Balance	1,144,719	1,186,655	1,187,404	1,336,579	1,144,719	1,454,003
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted Average Exercise Prices, Outstanding, Beginning Balance	$ 33.99	$ 34.00	$ 32.03	$ 31.16	$ 31.16
Weighted Average Exercise Price, Canceled				23.75
Weighted Average Exercise Prices, Forfeited	39.81		47.36
Weighted Average Exercise Prices, Exercised	11.49	19.48	14.68	20.78
Weighted Average Exercise Prices, Outstanding, Ending Balance	$ 34.69	$ 33.99	$ 34.00	$ 32.03	$ 34.69	$ 31.16
Weighted Average Remaining Life in Years	3 years 8 months	3 years 10 months 24 days	4 years 2 months 12 days	4 years 3 months 17 days		4 years 4 months 24 days
Aggregate Intrinsic Value, Outstanding, Beginning Balance	$ 19	$ 30	$ 31	$ 33	$ 33
Aggregate Intrinsic Value, Exercised	1	0	6	3	10	$ 30	$ 19
Aggregate Intrinsic Value, Outstanding, Ending Balance	$ 19	$ 19	$ 30	$ 31	$ 19	$ 33