Debt - Additional Information (Detail) - USD ($)	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Interest rate under multi currency facility	LIBOR, NIBOR or CDOR plus 1.125%
Capitalization ratio, Maximum	60.00%
Capitalization ratio, Actual	16.30%
Funds available under revolving credit facility	$ 3,000,000,000
Outstanding letters of credit under various bilateral letter of credit facilities	471,000,000
Carrying value of Unsecured Senior Notes	2,482,000,000	$ 2,480,000,000
Unsecured Debt [Member] \| Fair Value, Inputs, Level 2 [Member]
Debt Instrument [Line Items]
Fair value of Unsecured Senior Notes	$ 2,287,000,000	$ 2,346,000,000
Canadian Dollar Offered Rate (CDOR) [Member]
Debt Instrument [Line Items]
Variable rate	1.125%
Five Year Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility, borrowing capacity	$ 3,000,000,000
Credit facility, maturity date	Jun. 27, 2022
Credit facility, extendable borrowing capacity	$ 4,000,000,000
Variable rate basis	LIBOR, NIBOR or CDOR plus
Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility, Period	5 years
Five Year Credit Facility
Debt Instrument [Line Items]
Credit facility, borrowing capacity	$ 3,000,000,000
Credit facility, Period	5 years
Borrowings under commercial paper	$ 0
Outstanding letters of credit issued	$ 0

Interest rate under multicurrency facility.

No definition available.

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- Definition

Line of credit facility covenant compliance debt to capitalization percentage.

+ References

No definition available.

+ Details

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- Definition

Line of credit facility covenant terms maximum debt to capitalization percentage.

+ References

No definition available.

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- Definition

Line of credit facility extended borrowing capacity.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Outstanding letters of credit under various bilateral committed letter of credit facilities.

+ References

No definition available.

+ Details

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- Definition

Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(2))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-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 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a)(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

+ Details

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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

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Period remaining on line of credit facility before it terminates, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

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- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16(a))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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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