Cash, Cash Equivalents, and Marketable Securities Summary of Contractual Maturities of Cash Equivalent and Marketable Fixed Income Securities (Detail) $ in Millions	Sep. 30, 2015 USD ($)
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	$ 9,623
Estimated Fair Value	9,623
Due within one year
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	8,053
Estimated Fair Value	8,054
Due after one year through five years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	1,252
Estimated Fair Value	1,252
Due after five years through ten years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	141
Estimated Fair Value	141
Due after ten years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	177
Estimated Fair Value	$ 176