Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits
Balance at beginning of period	$ 63	$ 54	$ 81
Increases related to prior year tax positions	2	19	10
Decreases related to prior year tax positions	(2)	(8)	(10)
Settlements	(8)	(8)	(14)
Foreign currency impact	0	6	(15)
Increases related to current year tax positions	0	1	2
Lapse of statute of limitations	(3)	(1)	0
Balance at end of period	$ 52	$ 63	$ 54