Supplementary Information - Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for doubtful accounts
Balance at beginning of year	$ 323	$ 327	$ 330
Additions	48	47
Amounts written off	(42)	(43)
Other	(10)	(7)
Balance at end of year	$ 323	$ 327