Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017
Assets
Investment securities, trading:	$ 94	$ 82
Liabilities
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0
Fair Value, Measurements, Recurring \| Level 1
Assets
Total	9,677	7,762
Liabilities
Total	0	0
Fair Value, Measurements, Recurring \| Level 2
Assets
Total	4,253	6,551
Liabilities
Total	38	98
Cash equivalents and restricted cash: \| Fair Value, Measurements, Recurring \| Level 1 \| Money market funds
Assets
Cash equivalents and restricted cash:	7,341	5,935
Cash equivalents and restricted cash: \| Fair Value, Measurements, Recurring \| Level 2 \| U.S. government-sponsored debt securities
Assets
Cash equivalents and restricted cash:	282	2,870
Investment securities, trading: \| Fair Value, Measurements, Recurring \| Level 1 \| Equity securities
Assets
Investment securities, trading:	94	82
Investment securities, available-for-sale: \| Fair Value, Measurements, Recurring \| Level 1 \| Equity securities
Assets
Investment securities, available-for-sale:	181	124
Investment securities, available-for-sale: \| Fair Value, Measurements, Recurring \| Level 1 \| U.S. Treasury securities
Assets
Investment securities, available-for-sale:	2,061	1,621
Investment securities, available-for-sale: \| Fair Value, Measurements, Recurring \| Level 2 \| U.S. government-sponsored debt securities
Assets
Investment securities, available-for-sale:	3,895	3,663
Prepaid and other current assets: \| Fair Value, Measurements, Recurring \| Level 2 \| Foreign exchange derivative instruments
Assets
Prepaid and other current assets:	76	18
Foreign exchange derivative instruments \| Accrued liabilities: \| Fair Value, Measurements, Recurring \| Level 2
Liabilities
Accrued liabilities:	$ 38	$ 98

Fair value portion of accrued expenses.

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