Income Taxes - Change in Liability for Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 39	$ 53
Increase for other tax positions of prior years	15	3
Increase from Acquisitions	0	3
Decrease for payments and settlements	0	(4)
Decrease related to tax positions taken in prior years	(19)	(16)
Balance at end of year	$ 35	$ 39