Accounts Receivable - Activity of Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 142	$ 164	$ 178
Additions	145	153	176
Deductions	(181)	(175)	(190)
Ending Balance	$ 106	$ 142	$ 164