Income Taxes - Reconciliation of Unrecognized Tax Benefits (Expenses) (Details) - USD ($) $ in Thousands	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]
Unrecognized tax benefits as of January 1	$ 1,006	$ 815	$ 983
Gross increase in unrecognized tax benefits - prior year tax positions	0	52	0
Gross increase (decrease) in unrecognized tax benefits due to currency fluctuations - prior year tax positions	(10)		(131)
Gross increase (decrease) in unrecognized tax benefits due to currency fluctuations - prior year tax positions		43
Gross increase in unrecognized tax benefits - current year tax positions	75	111	293
Settlements with taxing authorities	(180)	0	(330)
Lapse of statute of limitations	(222)	(15)	0
Unrecognized tax benefits as of December 31	$ 669	$ 1,006	$ 815