Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2019	Sep. 30, 2018
Assets
Investment securities, available-for-sale:	$ 7,200	$ 7,500
Liabilities
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0
Fair Value, Measurements, Recurring \| Level 1
Assets
Total	7,989	8,873
Liabilities
Total	0	0
Fair Value, Measurements, Recurring \| Level 2
Assets
Total	5,848	6,134
Liabilities
Total	44	22
Cash equivalents and restricted cash equivalents: \| Fair Value, Measurements, Recurring \| Level 1 \| Money market funds
Assets
Cash equivalents and restricted cash equivalents:	6,089	6,252
Cash equivalents and restricted cash equivalents: \| Fair Value, Measurements, Recurring \| Level 2 \| U.S. government-sponsored debt securities
Assets
Cash equivalents and restricted cash equivalents:	200	1,048
Investment securities: \| Fair Value, Measurements, Recurring \| Level 1 \| Marketable equity securities
Assets
Marketable equity securities	157	113
Investment securities: \| Fair Value, Measurements, Recurring \| Level 1 \| U.S. Treasury securities
Assets
Investment securities, available-for-sale:	1,743	2,508
Investment securities: \| Fair Value, Measurements, Recurring \| Level 2 \| U.S. government-sponsored debt securities
Assets
Investment securities, available-for-sale:	5,482	5,008
Other current and non-current assets: \| Fair Value, Measurements, Recurring \| Level 2 \| Derivative instruments
Assets
Derivative Asset	166	78
Derivative instruments \| Accrued and other liabilities: \| Fair Value, Measurements, Recurring \| Level 2
Liabilities
Accrued Liabilities, Fair Value Disclosure	$ 44	$ 22

Fair value portion of accrued expenses.

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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 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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