Long-Term Debt - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Debt Disclosure [Abstract]
Interest paid, net of amounts capitalized		$ 204	$ 171
Maximum borrowing capacity under credit facility		4,000
Senior Notes \| Series X Notes, interest rate of 4.0%, face amount of $450, maturing April 15, 2028 (effective interest rate of 4.2%)
Debt Instrument [Line Items]
Senior notes, face amount	$ 450	$ 450
Interest rate	4.00%	4.00%
Proceeds from debt offering	$ 443

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Face (par) amount of debt instrument at time of issuance.

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Total interest costs incurred during the period and either capitalized or charged against earnings.

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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.

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The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination.

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