Summary of Contractual Maturities of Cash Equivalent and Marketable Fixed Income Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	$ 5,828
Estimated Fair Value	5,840
Due within one year
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	3,565
Estimated Fair Value	3,568
Due after one year through five years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	1,986
Estimated Fair Value	1,994
Due After Five Years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	277
Estimated Fair Value	$ 278