Fair Value Measurements and Investments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Assets
U.S. securities	$ 3,400	$ 6,300
Level 1 \| Fair Value, Measurements, Recurring
Assets
Marketable equity securities	121	126
Total	9,190	7,295
Liabilities
Deferred compensation liability	111	113
Total	111	113
Level 1 \| U.S. Treasury securities \| Fair Value, Measurements, Recurring
Assets
U.S. securities	205	675
Level 2 \| Fair Value, Measurements, Recurring
Assets
Derivative instruments	768	437
Total	3,917	6,179
Liabilities
Derivative instruments	272	52
Total	272	52
Level 2 \| U.S. government-sponsored debt securities \| Fair Value, Measurements, Recurring
Assets
U.S. securities	3,149	5,592
Money market funds \| Level 1 \| Fair Value, Measurements, Recurring
Assets
Cash equivalents and restricted cash equivalents:	8,864	6,494
U.S. government-sponsored debt securities \| Level 2 \| Fair Value, Measurements, Recurring
Assets
Cash equivalents and restricted cash equivalents:	$ 0	$ 150

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 asset or other 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 assets not subject to a master netting arrangement and not elected to be offset.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Fair value of financial and nonfinancial obligations.

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