Fair Value Measurements and Investments - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)	Jun. 30, 2019	Sep. 30, 2018
Assets
U.S. securities	$ 6,100,000,000	$ 7,500,000,000
Liabilities
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0
Level 1 \| Fair Value, Measurements, Recurring
Assets
Total	6,974,000,000	8,873,000,000
Liabilities
Total	0	0
Level 1 \| Marketable equity securities \| Fair Value, Measurements, Recurring
Assets
Marketable equity securities	125,000,000	113,000,000
Level 1 \| U.S. Treasury securities \| Fair Value, Measurements, Recurring
Assets
U.S. securities	574,000,000	2,508,000,000
Level 2 \| Fair Value, Measurements, Recurring
Assets
Total	5,745,000,000	6,134,000,000
Liabilities
Total	74,000,000	22,000,000
Level 2 \| Derivative instruments \| Fair Value, Measurements, Recurring
Assets
Derivative instruments	211,000,000	78,000,000
Liabilities
Derivative instruments	74,000,000	22,000,000
Level 2 \| U.S. government-sponsored debt securities \| Fair Value, Measurements, Recurring
Assets
U.S. securities	5,484,000,000	5,008,000,000
Money market funds \| Level 1 \| Fair Value, Measurements, Recurring
Assets
Cash equivalents and restricted cash equivalents:	6,275,000,000	6,252,000,000
U.S. government-sponsored debt securities \| Level 2 \| Fair Value, Measurements, Recurring
Assets
Cash equivalents and restricted cash equivalents:	$ 50,000,000	$ 1,048,000,000

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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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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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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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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