Fair Value Measurements and Investments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020
Assets
Investment securities	$ 2,000	$ 3,800
Level 1 \| Recurring
Assets
Total	15,421	14,573
Liabilities
Deferred compensation liability	159	135
Derivative instruments	0	0
Total	159	135
Level 1 \| Money market funds \| Recurring
Assets
Cash equivalents and restricted cash equivalents	13,702	12,522
Other current and non-current assets	2	0
Level 1 \| Marketable equity securities \| Recurring
Assets
Investment securities	191	148
Level 1 \| U.S. government-sponsored debt securities \| Recurring
Assets
Cash equivalents and restricted cash equivalents	0	0
Investment securities	0	0
Level 1 \| U.S. Treasury securities \| Recurring
Assets
Cash equivalents and restricted cash equivalents	450	650
Investment securities	1,076	1,253
Level 1 \| Derivative instruments \| Recurring
Assets
Other current and non-current assets	0	0
Level 2 \| Recurring
Assets
Total	1,925	4,563
Liabilities
Deferred compensation liability	0	0
Derivative instruments	201	181
Total	201	181
Level 2 \| Money market funds \| Recurring
Assets
Cash equivalents and restricted cash equivalents	0	0
Other current and non-current assets	0	0
Level 2 \| Marketable equity securities \| Recurring
Assets
Investment securities	0	0
Level 2 \| U.S. government-sponsored debt securities \| Recurring
Assets
Cash equivalents and restricted cash equivalents	600	1,469
Investment securities	901	2,582
Level 2 \| U.S. Treasury securities \| Recurring
Assets
Cash equivalents and restricted cash equivalents	0	0
Investment securities	0	0
Level 2 \| Derivative instruments \| Recurring
Assets
Other current and non-current assets	$ 424	$ 512

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, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value of financial and nonfinancial obligations.

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Fair value portion of other assets.

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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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