Debt - Senior Notes (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2017	Sep. 30, 2016
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	$ 1,749	$ 0
Carrying amount (long-term)	16,618	15,882
Principal amount	$ 18,500
2017 Notes
Debt Instrument [Line Items]
Stated interest rate (percent)	1.20%
2020 Notes
Debt Instrument [Line Items]
Stated interest rate (percent)	2.20%
September 2022 Notes
Debt Instrument [Line Items]
Stated interest rate (percent)	2.15%
2022 Notes
Debt Instrument [Line Items]
Stated interest rate (percent)	2.80%
2025 Notes
Debt Instrument [Line Items]
Stated interest rate (percent)	3.15%
2027 Notes
Debt Instrument [Line Items]
Stated interest rate (percent)	2.75%
2035 Notes
Debt Instrument [Line Items]
Stated interest rate (percent)	4.15%
2045 Notes
Debt Instrument [Line Items]
Stated interest rate (percent)	4.30%
2047 Notes
Debt Instrument [Line Items]
Stated interest rate (percent)	3.65%
Senior Notes
Debt Instrument [Line Items]
long-term debt, current maturities, gross	$ 1,750
Principal amount	16,750	16,000
Unamortized discounts and debt issuance costs (current maturities)	(1)
Long-term Debt, Current Maturities	1,749
Unamortized discounts and debt issuance costs (long-term)	(132)	(118)
Carrying amount (long-term)	16,618	15,882
Principal amount	18,500	16,000
Unamortized discounts and debt issuance costs	133	118
Carrying amount	18,367	15,882
Senior Notes \| 2017 Notes
Debt Instrument [Line Items]
long-term debt, current maturities, gross	1,750
Principal amount		1,750
Unamortized discounts and debt issuance costs (current maturities)	(1)
Long-term Debt, Current Maturities	$ 1,749
Unamortized discounts and debt issuance costs (long-term)		(4)
Carrying amount (long-term)		1,746
Effective interest rate (percent)	1.37%
Stated interest rate (percent)	1.20%
Redemption price, additional basis spread (percent)	0.05%
Senior Notes \| 2020 Notes
Debt Instrument [Line Items]
Principal amount	$ 3,000	3,000
Unamortized discounts and debt issuance costs (long-term)	(10)	(12)
Carrying amount (long-term)	$ 2,990	2,988
Effective interest rate (percent)	2.30%
Stated interest rate (percent)	2.20%
Redemption price, additional basis spread (percent)	0.10%
Senior Notes \| September 2022 Notes
Debt Instrument [Line Items]
Principal amount	$ 1,000	0
Unamortized discounts and debt issuance costs (long-term)	(7)	0
Carrying amount (long-term)	$ 993	0
Effective interest rate (percent)	2.30%
Stated interest rate (percent)	2.15%
Redemption price, additional basis spread (percent)	0.10%
Senior Notes \| 2022 Notes
Debt Instrument [Line Items]
Principal amount	$ 2,250	2,250
Unamortized discounts and debt issuance costs (long-term)	(10)	(12)
Carrying amount (long-term)	$ 2,240	2,238
Effective interest rate (percent)	2.89%
Stated interest rate (percent)	2.80%
Redemption price, additional basis spread (percent)	0.125%
Senior Notes \| 2025 Notes
Debt Instrument [Line Items]
Principal amount	$ 4,000	4,000
Unamortized discounts and debt issuance costs (long-term)	(33)	(36)
Carrying amount (long-term)	$ 3,967	3,964
Effective interest rate (percent)	3.26%
Stated interest rate (percent)	3.15%
Redemption price, additional basis spread (percent)	0.15%
Senior Notes \| 2027 Notes
Debt Instrument [Line Items]
Principal amount	$ 750	0
Unamortized discounts and debt issuance costs (long-term)	(10)	0
Carrying amount (long-term)	$ 740	0
Effective interest rate (percent)	2.91%
Stated interest rate (percent)	2.75%
Redemption price, additional basis spread (percent)	0.125%
Senior Notes \| 2035 Notes
Debt Instrument [Line Items]
Principal amount	$ 1,500	1,500
Unamortized discounts and debt issuance costs (long-term)	(15)	(15)
Carrying amount (long-term)	$ 1,485	1,485
Effective interest rate (percent)	4.23%
Stated interest rate (percent)	4.15%
Redemption price, additional basis spread (percent)	0.20%
Senior Notes \| 2045 Notes
Debt Instrument [Line Items]
Principal amount	$ 3,500	3,500
Unamortized discounts and debt issuance costs (long-term)	(37)	(39)
Carrying amount (long-term)	$ 3,463	3,461
Effective interest rate (percent)	4.37%
Stated interest rate (percent)	4.30%
Redemption price, additional basis spread (percent)	0.20%
Senior Notes \| 2047 Notes
Debt Instrument [Line Items]
Principal amount	$ 750	0
Unamortized discounts and debt issuance costs (long-term)	(10)	0
Carrying amount (long-term)	$ 740	$ 0
Effective interest rate (percent)	3.73%
Stated interest rate (percent)	3.65%
Redemption price, additional basis spread (percent)	0.15%

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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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, after accumulated amortization, of debt issuance costs classified as current. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

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Amount, after accumulated amortization, of debt issuance costs classified as noncurrent. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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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Debt Instrument, Redemption Price, Basis Spread in Excess of Principal

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long-term debt, current maturities, gross

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