Summary of Contractual Maturities of Cash Equivalent and Marketable Fixed-Income Securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	$ 8,509
Estimated Fair Value	8,524
Due within one year
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	6,011
Estimated Fair Value	6,012
Due after one year through five years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	2,498
Estimated Fair Value	$ 2,512