Debt - Summary of Debt (Details) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020
Debt Instrument [Line Items]
Unamortized discounts and debt issuance costs	$ (170)	$ (178)
Hedge accounting fair value adjustments	144	248
Total carrying value of debt	20,974	24,070
Current maturities of debt	0	2,999
Long-term debt	20,974	21,071
Senior Notes
Debt Instrument [Line Items]
Total debt	21,000	24,000
Total carrying value of debt	21,000	24,100
Senior Notes \| 2.20% Senior Notes due December 2020
Debt Instrument [Line Items]
Total debt	$ 0	3,000
Effective interest rate (percent)	2.30%
Stated interest rate (percent)	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 (percent)	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 (percent)	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 (percent)	3.15%
Senior Notes \| 1.90% Senior Notes due April 2027
Debt Instrument [Line Items]
Total debt	$ 1,500	1,500
Effective interest rate (percent)	2.02%
Stated interest rate (percent)	1.90%
Senior Notes \| 0.75% Senior Notes due August 2027
Debt Instrument [Line Items]
Total debt	$ 500	500
Effective interest rate (percent)	0.84%
Stated interest rate (percent)	0.75%
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 (percent)	2.75%
Senior Notes \| 2.05% Senior Notes due April 2030
Debt Instrument [Line Items]
Total debt	$ 1,500	1,500
Effective interest rate (percent)	2.13%
Stated interest rate (percent)	2.05%
Senior Notes \| 1.10% Senior Notes due February 2031
Debt Instrument [Line Items]
Total debt	$ 1,000	1,000
Effective interest rate (percent)	1.20%
Stated interest rate (percent)	1.10%
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 (percent)	4.15%
Senior Notes \| 2.70% Senior Notes due April 2040
Debt Instrument [Line Items]
Total debt	$ 1,000	1,000
Effective interest rate (percent)	2.80%
Stated interest rate (percent)	2.70%
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 (percent)	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 (percent)	3.65%
Senior Notes \| 2.00% Senior Notes due August 2050
Debt Instrument [Line Items]
Total debt	$ 1,750	$ 1,750
Effective interest rate (percent)	2.09%
Stated interest rate (percent)	2.00%

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 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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