Debt - Debt (Details) - USD ($) $ in Millions	Mar. 31, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Long-term Debt	$ 16,630	$ 16,630
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	(114)	(120)
Hedged Liability, Fair Value Hedge, Cumulative Increase (Decrease)	(6)	0
Senior Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	16,750	16,750
Long-term Debt	16,600	16,600
Senior Notes \| 2020 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 3,000	3,000
Effective Interest Rate (percent)	2.30%
Debt Instrument, Interest Rate, Stated Percentage	2.20%
Senior Notes \| September 2022 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 1,000	1,000
Effective Interest Rate (percent)	2.30%
Debt Instrument, Interest Rate, Stated Percentage	2.15%
Senior Notes \| December 2022 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 2,250	2,250
Effective Interest Rate (percent)	2.89%
Debt Instrument, Interest Rate, Stated Percentage	2.80%
Senior Notes \| 2025 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 4,000	4,000
Effective Interest Rate (percent)	3.26%
Debt Instrument, Interest Rate, Stated Percentage	3.15%
Senior Notes \| 2027 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 750	750
Effective Interest Rate (percent)	2.91%
Debt Instrument, Interest Rate, Stated Percentage	2.75%
Senior Notes \| 2035 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 1,500	1,500
Effective Interest Rate (percent)	4.23%
Debt Instrument, Interest Rate, Stated Percentage	4.15%
Senior Notes \| 2045 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 3,500	3,500
Effective Interest Rate (percent)	4.37%
Debt Instrument, Interest Rate, Stated Percentage	4.30%
Senior Notes \| 2047 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 750	$ 750
Effective Interest Rate (percent)	3.73%
Debt Instrument, Interest Rate, Stated Percentage	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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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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