Fair Value Measurements (Losses) gains on forward currency contracts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Losses on foreign exchange forward contracts	$ (2,209)	$ (1,647)	$ (10,787)