Amortized Cost and Estimated Fair Value of Marketable Securities by Contractual Maturity (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Cost
Due in one year or less	$ 410
Due after one year through three years	431
Due after three years through five years	19
Due after five years	111
Marketable Securities, Debt Maturities, Amortized Cost, Total	971
Equity securities	89
Total Amortized Cost	1,060	$ 1,083
Estimated Fair Value
Due in one year or less	410
Due after one year through three years	428
Due after three years through five years	19
Due after five years	113
Marketable Securities, Debt Maturities, Fair Value, Total	970
Equity securities	90
Total Estimated Fair Value	$ 1,060	$ 1,091