Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017
Assets
Investment securities, trading:	$ 106	$ 82
Liabilities
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0
Fair Value, Measurements, Recurring \| Level 1
Assets
Total	8,475	7,762
Liabilities
Total	0	0
Fair Value, Measurements, Recurring \| Level 2
Assets
Total	4,123	6,551
Liabilities
Total	65	98
Fair Value, Measurements, Recurring \| Level 2 \| U.S. government-sponsored debt securities
Assets
Investment securities, available-for-sale:	3,530	3,663
Cash equivalents and restricted cash: \| Fair Value, Measurements, Recurring \| Level 1 \| Money market funds
Assets
Cash equivalents and restricted cash:	5,918	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:	567	2,870
Investment securities, trading: \| Fair Value, Measurements, Recurring \| Level 1 \| Equity securities
Assets
Investment securities, trading:	106	82
Investment securities, available-for-sale: \| Fair Value, Measurements, Recurring \| Level 1 \| Equity securities
Assets
Investment securities, available-for-sale:	114	124
Investment securities, available-for-sale: \| Fair Value, Measurements, Recurring \| Level 1 \| U.S. Treasury securities
Assets
Investment securities, available-for-sale:	2,337	1,621
Prepaid and other current assets: \| Fair Value, Measurements, Recurring \| Level 2 \| Foreign exchange derivative instruments
Assets
Prepaid and other current assets:	26	18
Foreign exchange derivative instruments \| Accrued liabilities: \| Fair Value, Measurements, Recurring \| Level 2
Liabilities
Accrued liabilities:	$ 65	$ 98

Fair value portion of accrued expenses.

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