Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for Doubtful Accounts [Roll Forward]
Balance, beginning of period	$ 129	$ 113	$ 124
Charged to expense	659	570	469
Uncollected balances written off, net of recoveries	(637)	(554)	(480)
Balance, end of period	$ 151	$ 129	$ 113