Supplementary Information - Change in Allowance for Doubtful Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting [Abstract]
Balance at beginning of year	$ 255	$ 249	$ 241
Additions	58	5	15
Amounts written off	(12)	1	(7)
Balance at end of year	$ 301	$ 255	$ 249