Cash, Cash Equivalents, and Marketable Securities Summary of Contractual Maturities of Cash Equivalent and Marketable Fixed Income Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	$ 10,013
Estimated Fair Value	10,015
Due within one year
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	8,552
Estimated Fair Value	8,552
Due after one year through five years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	1,135
Estimated Fair Value	1,137
Due after five years through ten years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	148
Estimated Fair Value	148
Due after ten years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	178
Estimated Fair Value	$ 178