Cash, Cash Equivalents, and Marketable Securities Summary of Contractual Maturities of Cash Equivalent and Marketable Fixed Income Securities (Detail) $ in Millions	Mar. 31, 2017 USD ($)
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	$ 15,974
Estimated Fair Value	15,967
Due within one year
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	13,191
Estimated Fair Value	13,190
Due after one year through five years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	2,411
Estimated Fair Value	2,407
Due after five years through ten years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	197
Estimated Fair Value	196
Due after ten years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	175
Estimated Fair Value	$ 174