Income Taxes - Uncertain Tax Positions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 261	$ 220	$ 171
Increases to tax positions related to the current year	24	35	43
Increases to tax positions related to prior years	2	4	8
Decreases to tax positions related to prior years	0	1	2
Reductions due to lapsed statute of limitations	2	3	5
Settlements during current year	0	1	0
Interest and penalties	8	7	5
Ending balance	$ 293	$ 261	$ 220