Schedule II - Valuation and Qualifying Accounts (Details) - Allowance for doubtful accounts - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Valuation Allowances [Roll Forward]
Balance at beginning of year	$ 33	$ 28	$ 37
Net charges to income	21	15	8
Deductions and other	(20)	(11)	(17)
Balance at end of year	$ 34	$ 33	$ 28