Amortized Cost and Estimated Fair Value of Marketable Securities by Contractual Maturity (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Cost
Due in one year or less	$ 1,267
Due after one year through three years	422
Due after three years through five years	15
Due after five years	103
Marketable Securities, Debt Maturities, Amortized Cost, Total	1,807
Equity securities	123
Total Amortized Cost	1,930	992
Estimated Fair Value
Due in one year or less	1,267
Due after one year through three years	423
Due after three years through five years	16
Due after five years	105
Marketable Securities, Debt Maturities, Fair Value, Total	1,811
Equity securities	124
Total Estimated Fair Value	$ 1,935	$ 992