Investments in Unconsolidated Affiliates - Equity Method Investment Summarized Financial Information, Balance Sheet (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Equity Method Investments and Joint Ventures [Abstract]
Current assets	$ 346		$ 346		$ 463
Long-term assets	7,609		7,609		7,546
Current liabilities	(147)		(147)		(231)
Long-term liabilities	(259)		(259)		(252)
Net assets	7,549		7,549		$ 7,526
Operating revenue	510	$ 432	1,037	$ 853
Operating expenses	191	162	376	353
Net income	$ 318	$ 269	$ 659	$ 500