Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2017	Sep. 30, 2016
Investment securities:
Trading	$ 80	$ 71
Accrued liabilities
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0
Level 1 | Fair Value, Measurements, Recurring
Prepaid and other current assets:
Fair value, total assets	7,417	6,839
Accrued liabilities
Fair value, total liabilities	0	0
Level 2 | Fair Value, Measurements, Recurring
Prepaid and other current assets:
Fair value, total assets	3,983	5,200
Accrued liabilities
Fair value, total liabilities	80	136
Foreign exchange derivative instruments | Level 2 | Fair Value, Measurements, Recurring
Accrued liabilities
Foreign exchange derivative instruments	80	116
Other liabilities	0	20
Money market funds | Level 1 | Fair Value, Measurements, Recurring
Cash equivalents and restricted cash:
Cash equivalents and restricted cash:	5,860	4,537
Equity securities | Level 1 | Fair Value, Measurements, Recurring
Investment securities:
Trading	80	71
Available-for-sale securities	104	53
U.S. Treasury securities | Level 1 | Fair Value, Measurements, Recurring
Investment securities:
Available-for-sale securities	1,373	2,178
U.S. government-sponsored debt securities | Level 2 | Fair Value, Measurements, Recurring
Cash equivalents and restricted cash:
Cash equivalents and restricted cash:	626	196
Investment securities:
Available-for-sale securities	3,234	4,699
Corporate debt securities | Level 2 | Fair Value, Measurements, Recurring
Investment securities:
Available-for-sale securities	80	249
Foreign exchange derivative instruments | Level 2 | Fair Value, Measurements, Recurring
Prepaid and other current assets:
Prepaid and other current assets:	43	50
Other assets	$ 0	$ 6