Debt - Summary of Debt (Details) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Total senior notes	$ 16,750	$ 16,750
Unamortized discounts and debt issuance costs	(111)	(120)
Hedge accounting fair value adjustments	55	0
Senior Notes
Debt Instrument [Line Items]
Total long-term debt	16,694	16,630
Senior Notes \| 2.20% Senior Notes due December 2020
Debt Instrument [Line Items]
Total senior notes	$ 3,000	3,000
Effective interest rate (percent)	2.30%
Stated interest rate	2.20%
Senior Notes \| 2.15% Senior Notes due September 2022
Debt Instrument [Line Items]
Total senior notes	$ 1,000	1,000
Effective interest rate (percent)	2.30%
Stated interest rate	2.15%
Senior Notes \| 2.80% Senior Notes due December 2022
Debt Instrument [Line Items]
Total senior notes	$ 2,250	2,250
Effective interest rate (percent)	2.89%
Stated interest rate	2.80%
Senior Notes \| 3.15% Senior Notes due December 2025
Debt Instrument [Line Items]
Total senior notes	$ 4,000	4,000
Effective interest rate (percent)	3.26%
Stated interest rate	3.15%
Senior Notes \| 2.75% Senior Notes due September 2027
Debt Instrument [Line Items]
Total senior notes	$ 750	750
Effective interest rate (percent)	2.91%
Stated interest rate	2.75%
Senior Notes \| 4.15% Senior Notes due December 2035
Debt Instrument [Line Items]
Total senior notes	$ 1,500	1,500
Effective interest rate (percent)	4.23%
Stated interest rate	4.15%
Senior Notes \| 4.30% Senior Notes due December 2045
Debt Instrument [Line Items]
Total senior notes	$ 3,500	3,500
Effective interest rate (percent)	4.37%
Stated interest rate	4.30%
Senior Notes \| 3.65% Senior Notes due September 2047
Debt Instrument [Line Items]
Total senior notes	$ 750	$ 750
Effective interest rate (percent)	3.73%
Stated interest rate	3.65%

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of unamortized debt discount (premium) and debt issuance costs.

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- Definition

Amount of cumulative increase (decrease) in fair value of hedged liability in fair value hedge, attributable to hedged risk.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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