Debt - Debt (Details) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	$ 0	$ 1,749
Carrying Amount, noncurrent	16,627	$ 16,618
2017 Notes
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		1.20%
Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount, current maturities	0	$ 1,750
Unamortized Discounts and Debt Issuance Costs, current maturities	0	(1)
Long-term Debt, Current Maturities	0	1,749
Principal Amount, noncurrent	16,750	16,750
Unamortized Discounts and Debt Issuance Costs, noncurrent	(123)	(132)
Carrying Amount, noncurrent	16,627	16,618
Total debt, principal	16,750	18,500
Unamortized Discounts and Debt Issuance Costs	(123)	(133)
Carrying Amount	16,627	18,367
Senior Notes [Member] \| 2017 Notes
Debt Instrument [Line Items]
Principal Amount, current maturities	0	1,750
Unamortized Discounts and Debt Issuance Costs, current maturities	0	(1)
Long-term Debt, Current Maturities	0	$ 1,749
Effective Interest Rate (percent)		1.37%
Debt Instrument, Interest Rate, Stated Percentage		1.20%
Senior Notes [Member] \| 2020 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	3,000	$ 3,000
Unamortized Discounts and Debt Issuance Costs, noncurrent	(7)	(10)
Carrying Amount, noncurrent	$ 2,993	2,990
Effective Interest Rate (percent)	2.30%
Debt Instrument, Interest Rate, Stated Percentage	2.20%
Senior Notes [Member] \| September 2022 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 1,000	1,000
Unamortized Discounts and Debt Issuance Costs, noncurrent	(6)	(7)
Carrying Amount, noncurrent	$ 994	993
Effective Interest Rate (percent)	2.30%
Debt Instrument, Interest Rate, Stated Percentage	2.15%
Senior Notes [Member] \| 2022 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 2,250	2,250
Unamortized Discounts and Debt Issuance Costs, noncurrent	(9)	(10)
Carrying Amount, noncurrent	$ 2,241	2,240
Effective Interest Rate (percent)	2.89%
Debt Instrument, Interest Rate, Stated Percentage	2.80%
Senior Notes [Member] \| 2025 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 4,000	4,000
Unamortized Discounts and Debt Issuance Costs, noncurrent	(31)	(33)
Carrying Amount, noncurrent	$ 3,969	3,967
Effective Interest Rate (percent)	3.26%
Debt Instrument, Interest Rate, Stated Percentage	3.15%
Senior Notes [Member] \| 2027 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 750	750
Unamortized Discounts and Debt Issuance Costs, noncurrent	(9)	(10)
Carrying Amount, noncurrent	$ 741	740
Effective Interest Rate (percent)	2.91%
Debt Instrument, Interest Rate, Stated Percentage	2.75%
Senior Notes [Member] \| 2035 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 1,500	1,500
Unamortized Discounts and Debt Issuance Costs, noncurrent	(14)	(15)
Carrying Amount, noncurrent	$ 1,486	1,485
Effective Interest Rate (percent)	4.23%
Debt Instrument, Interest Rate, Stated Percentage	4.15%
Senior Notes [Member] \| 2045 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 3,500	3,500
Unamortized Discounts and Debt Issuance Costs, noncurrent	(37)	(37)
Carrying Amount, noncurrent	$ 3,463	3,463
Effective Interest Rate (percent)	4.37%
Debt Instrument, Interest Rate, Stated Percentage	4.30%
Senior Notes [Member] \| 2047 Notes
Debt Instrument [Line Items]
Principal Amount, noncurrent	$ 750	750
Unamortized Discounts and Debt Issuance Costs, noncurrent	(10)	(10)
Carrying Amount, noncurrent	$ 740	$ 740
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.

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
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-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
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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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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Represents the aggregate of total long-term debt, including current maturities and short-term debt.

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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
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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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, 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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Debt, Long-term and Short-term, Gross

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