Debt - Summary of Debt (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Unamortized discounts and debt issuance costs	$ (103)	$ (108)
Hedge accounting fair value adjustments	243	87
Total carrying value of debt	17,892	16,729
Current maturities of debt	3,999	0
Long-term debt	13,893	16,729
Senior Notes
Debt Instrument [Line Items]
Total debt	17,752	16,750
Total carrying value of debt	16,900	16,700
Senior Notes \| 2.20% Senior Notes due December 2020
Debt Instrument [Line Items]
Total debt	$ 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 debt	$ 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 debt	$ 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 debt	$ 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 debt	$ 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 debt	$ 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 debt	$ 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 debt	$ 750	750
Effective interest rate (percent)	3.73%
Stated interest rate	3.65%
Commercial Paper
Debt Instrument [Line Items]
Short-term Debt	$ 1,002	$ 0
Effective interest rate (percent)	1.55%

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

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

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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