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Line of credit facility extended borrowing capacity.

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

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Payment on redemption unsecured debt.

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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.

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Debt - Additional Information (Detail) - USD ($)				12 Months Ended
Debt - Additional Information (Detail) - USD ($)	Dec. 04, 2019	Nov. 04, 2019	Oct. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
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				22.40%
Funds available under revolving credit facility				$ 2,000,000,000.0
Loss on extinguishment of debt				(26,000,000)
Outstanding letters of credit under various bilateral letter of credit facilities				502,000,000
Carrying value of Unsecured Senior Notes				1,980,000,000	$ 2,482,000,000
Fair Value, Inputs, Level 2 [Member] \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Fair value of Unsecured Senior Notes				$ 1,947,000,000	$ 2,211,000,000
3.60% unsecured Senior Notes due 2029 [Member]
Debt Instrument [Line Items]
Unsecured senior notes issued		$ 500,000,000
Senior notes interest rate		3.60%
Proceeds from issuance of unsecured senior notes		$ 493,000,000
Underwriting expense		3,000,000
Discount on unsecured senior notes		$ 4,000,000
Description of Unsecured senior notes				Interest on each series of notes is due on June 1 and December 1 of each year, beginning on June 1, 2020. The Company may redeem some or all of the Senior Notes at any time at the applicable redemption price, plus accrued interest, if any, to the redemption date. At December 31, 2020, the Company was in compliance with the covenants under the indenture governing the Senior Notes.
2.60% unsecured Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Senior notes interest rate	2.60%
Repayment of debt	$ 1,000,000,000
Payment on redemption	1,023,000,000
Premium on redemption	23,100,000
Accrued and unpaid interest	200,000
Loss on extinguishment of debt	(26,000,000)
Premium on debt	23,000,000
Non cash charges	$ 3,000,000
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			$ 2,000,000,000
Credit facility, maturity date				Oct. 30, 2024
Credit facility, extendable borrowing capacity			$ 3,000,000,000.0
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 [Member]
Debt Instrument [Line Items]
Credit facility, borrowing capacity				$ 2,000,000,000.0
Credit facility, Period					5 years
Borrowings under commercial paper				0
Outstanding letters of credit issued				$ 0