Cash, Cash Equivalents, and Marketable Securities Summary of Contractual Maturities of Cash Equivalent and Marketable Fixed Income Securities (Detail) $ in Millions	Sep. 30, 2016 USD ($)
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	$ 12,955
Estimated Fair Value	12,956
Due within one year
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	10,647
Estimated Fair Value	10,648
Due after one year through five years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	1,933
Estimated Fair Value	1,934
Due after five years through ten years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	193
Estimated Fair Value	193
Due after ten years
Investments Classified by Contractual Maturity Date [Line Items]
Amortized Cost	182
Estimated Fair Value	$ 181