Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2016	Sep. 30, 2016
Investment securities:
Trading	$ 82	$ 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	7,517	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	4,891	5,200
Accrued liabilities
Fair value, total liabilities	84	136
Foreign exchange derivative instruments | Level 2 | Fair Value, Measurements, Recurring
Accrued liabilities
Foreign exchange derivative instruments	76	116
Other liabilities	8	20
Money market funds | Level 1 | Fair Value, Measurements, Recurring
Cash equivalents and restricted cash:
Cash equivalents and restricted cash:	4,819	4,537
Equity securities | Level 1 | Fair Value, Measurements, Recurring
Investment securities:
Trading	82	71
Available-for-sale securities	62	53
U.S. Treasury securities | Level 1 | Fair Value, Measurements, Recurring
Investment securities:
Available-for-sale securities	2,554	2,178
U.S. government-sponsored debt securities | Level 2 | Fair Value, Measurements, Recurring
Cash equivalents and restricted cash:
Cash equivalents and restricted cash:	0	196
Investment securities:
Available-for-sale securities	4,671	4,699
Corporate debt securities | Level 2 | Fair Value, Measurements, Recurring
Investment securities:
Available-for-sale securities	130	249
Foreign exchange derivative instruments | Level 2 | Fair Value, Measurements, Recurring
Prepaid and other current assets:
Prepaid and other current assets:	88	50
Other assets	$ 2	$ 6