Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Accounting Policies [Abstract]
Receivables from contracts with customers	$ 253		$ 253		$ 205
Unbilled revenues	78		78		$ 61
Franchise fees and gross receipts taxes	$ 28	$ 32	$ 81	$ 88