Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-09, Allowance, Credit Loss [Member]
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	$ 442	$ 576	$ 637
Additions	5	96	173
Write-offs, net of recoveries	(123)	(230)	(234)
Balance at end of year	324	442	576
SEC Schedule, 12-09, Valuation Allowance, Deferred Tax Asset [Member]
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	4,690	4,197	3,302
Additions	105	510	907
Deductions	(26)	(17)	(12)
Balance at end of year	$ 4,769	$ 4,690	$ 4,197