Income Taxes - Reconciliation of Beginning and Ending Balances of Total Amounts of Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 56	$ 57	$ 77
Additions to prior years' tax positions	0	4	0
Reductions to prior years' tax positions	(1)	(6)	(4)
Additions to current year tax positions	4	2	0
Reductions in tax positions due to lapse of statutory limitations	(5)	(4)	(7)
Settlements	0	0	(2)
Foreign currency translation	0	3
Foreign currency translation			(7)
Ending balance	$ 54	$ 56	$ 57