Debt (Details 1) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Total Long-term Debt	$ 24,153	$ 20,512
Year-End Stated Rate on Debt	1.51%	1.13%
Unamortized Underwriting Fees	$ (49)	$ (39)
Long-term Debt [Member]
Debt Instrument [Line Items]
Year-End Stated Rate on Debt	2.21%	1.88%
Fixed Rate Senior Notes Amount [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, Start	2018
Debt Instrument, Maturity Date Range, End	2027
Total Long-term Debt	$ 19,652	$ 16,201
Year-End Stated Rate on Debt	2.24%	1.98%
Year End Effective Interest Rate With Swaps	2.27%	1.44%
Floating Rate Senior Notes Amount [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, Start	2018
Debt Instrument, Maturity Date Range, End	2022
Total Long-term Debt	$ 4,550	$ 4,350
Year-End Stated Rate on Debt	2.09%	1.52%
Year End Effective Interest Rate With Swaps	0.00%	0.00%

Latest date the outstanding debt instruments are required to be repaid.

No definition available.

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- Definition

Earliest date the outstanding debt instruments are required to be repaid.

+ References

No definition available.

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- Definition

Reflects as of the balance-sheet date the effective interest rates on debt outstanding presented if swaps are in place to hedge the underlying debt.

+ References

No definition available.

+ Details

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

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.

+ References

No definition available.

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- Definition

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: 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

Reference 3: 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

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- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

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 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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