Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Investment securities
Trading	$ 82	$ 71
Available-for-sale securities	5,408	7,179
Fair Value, Measurements, Recurring \| Level 1
Investment securities
Fair value, total assets	7,762	6,839
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	5,935	4,537
Fair Value, Measurements, Recurring \| Level 1 \| Equity securities
Investment securities
Trading	82	71
Available-for-sale securities	124	53
Fair Value, Measurements, Recurring \| Level 1 \| U.S. Treasury securities
Investment securities
Available-for-sale securities	1,621	2,178
Fair Value, Measurements, Recurring \| Level 2
Investment securities
Fair value, total assets	6,551	5,200
Liabilities
Fair value, total liabilities	98	136
Fair Value, Measurements, Recurring \| Level 2 \| U.S. government-sponsored debt securities
Cash equivalents and restricted cash
Cash equivalents and restricted cash	2,870	196
Investment securities
Available-for-sale securities	3,663	4,699
Fair Value, Measurements, Recurring \| Level 2 \| Corporate debt securities
Investment securities
Available-for-sale securities	0	249
Prepaid and other current assets \| Fair Value, Measurements, Recurring \| Level 2 \| Foreign exchange derivative instruments
Investment securities
Foreign exchange derivative instruments	18	50
Other Assets \| Fair Value, Measurements, Recurring \| Level 2 \| Foreign exchange derivative instruments
Investment securities
Foreign exchange derivative instruments		6
Accrued Liabilities \| Fair Value, Measurements, Recurring \| Level 2 \| Foreign exchange derivative instruments
Liabilities
Foreign exchange derivative instruments	98	116
Other Liabilities \| Fair Value, Measurements, Recurring \| Level 2 \| Foreign exchange derivative instruments
Liabilities
Foreign exchange derivative instruments	$ 0	$ 20

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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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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

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