Intrinsic and Fair Value of Stock-based Compensation (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value	[1]	$ 28	$ 43	$ 15
Share-based Compensation Arrangement by Share-based Payment Award, RSUs, Vested in Period, Fair Value	[1]	62	74	23
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value	[1]	$ 0	$ 41	$ 35

[1] Measured as of the grant date.