Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Investment securities
Trading	$ 98	$ 82
Available-for-sale securities	7,531	5,408
Fair Value, Measurements, Recurring \| Level 1
Investment securities
Fair value, total assets	8,873	7,762
Liabilities
Fair value, total liabilities	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Money market funds
Cash equivalents and restricted cash
Cash equivalents and restricted cash	6,252	5,935
Fair Value, Measurements, Recurring \| Level 1 \| Equity securities
Investment securities
Trading	98	82
Available-for-sale securities	15	124
Fair Value, Measurements, Recurring \| Level 1 \| U.S. Treasury securities
Investment securities
Available-for-sale securities	2,508	1,621
Fair Value, Measurements, Recurring \| Level 2
Investment securities
Fair value, total assets	6,134	6,551
Liabilities
Fair value, total liabilities	22	98
Fair Value, Measurements, Recurring \| Level 2 \| U.S. government-sponsored debt securities
Cash equivalents and restricted cash
Cash equivalents and restricted cash	1,048	2,870
Investment securities
Available-for-sale securities	5,008	3,663
Prepaid and other current assets \| Fair Value, Measurements, Recurring \| Level 2 \| Foreign exchange derivative instruments
Investment securities
Foreign exchange derivative instruments	78	18
Accrued Liabilities \| Fair Value, Measurements, Recurring \| Level 2 \| Foreign exchange derivative instruments
Liabilities
Foreign exchange derivative instruments	$ 22	$ 98

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Fair Value, Assets Measured on Recurring Basis. Short-term investments [Abstract]

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