Summary of Contractual Maturities of Cash Equivalent and Marketable Fixed-Income Securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	$ 8,934
Estimated Fair Value	8,950
Due within one year
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	6,689
Estimated Fair Value	6,691
Due after one year through five years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	1,968
Estimated Fair Value	1,981
Due After Five Years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	277
Estimated Fair Value	$ 278