Foreign Currency Derivative Instruments (Gains and losses on foreign currency derivative contracts partially offset by net gains and losses on underlying monetary assets) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Foreign Currency Derivatives [Abstract]
Total gain (loss)	$ 0	$ (37)	$ 179