Fair Value Measurements and Investments - Assets and Liabilities Measured (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Assets
U.S. securities	$ 3,835	$ 6,267
Fair Value, Measurements, Recurring \| Level 1
Assets
Total	14,573	7,295
Liabilities
Deferred compensation liability	135	113
Total	135	113
Fair Value, Measurements, Recurring \| Level 2
Assets
Total	4,563	6,179
Liabilities
Deferred compensation liability	0	0
Total	181	52
Money market funds \| Fair Value, Measurements, Recurring \| Level 1
Assets
Cash equivalents and restricted cash equivalents	12,522	6,494
Money market funds \| Fair Value, Measurements, Recurring \| Level 2
Assets
Cash equivalents and restricted cash equivalents	0	0
U.S. government-sponsored debt securities \| Fair Value, Measurements, Recurring \| Level 1
Assets
Cash equivalents and restricted cash equivalents	650	0
U.S. government-sponsored debt securities \| Fair Value, Measurements, Recurring \| Level 2
Assets
Cash equivalents and restricted cash equivalents	1,469	150
Equity securities \| Fair Value, Measurements, Recurring \| Level 1
Assets
Marketable equity securities	148	126
Equity securities \| Fair Value, Measurements, Recurring \| Level 2
Assets
Marketable equity securities	0	0
U.S. government-sponsored debt securities
Assets
U.S. securities	2,582	5,592
U.S. government-sponsored debt securities \| Fair Value, Measurements, Recurring \| Level 1
Assets
U.S. securities	0	0
U.S. government-sponsored debt securities \| Fair Value, Measurements, Recurring \| Level 2
Assets
U.S. securities	2,582	5,592
U.S. Treasury securities
Assets
U.S. securities	1,253	675
U.S. Treasury securities \| Fair Value, Measurements, Recurring \| Level 1
Assets
U.S. securities	1,253	675
U.S. Treasury securities \| Fair Value, Measurements, Recurring \| Level 2
Assets
U.S. securities	0	0
Derivative instruments \| Fair Value, Measurements, Recurring \| Level 1
Assets
Derivative instruments	0	0
Liabilities
Derivative instruments	0	0
Derivative instruments \| Fair Value, Measurements, Recurring \| Level 2
Assets
Derivative instruments	512	437
Liabilities
Derivative instruments	$ 181	$ 52

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 other comprehensive income (available-for-sale).

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value of financial and nonfinancial obligations.

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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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