Supplementary Information - Allowance for Credit Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance for doubtful accounts
Balance at beginning of year	$ 373	$ 323	$ 327
Provision	66	48
Write-offs & other	(16)	(52)
Balance at end of year	$ 373	$ 323