Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits
Balance at beginning of period	$ 52	$ 63	$ 54
Increases related to prior year tax positions	9	2	19
Decreases related to prior year tax positions	(1)	(2)	(8)
Settlements	(2)	(8)	(8)
Foreign currency impact	(5)	0
Foreign currency impact			6
Increases related to current year tax positions	21	0	1
Lapse of statute of limitations	(3)	(3)	(1)
Balance at end of period	$ 71	$ 52	$ 63