Fair Value Measurements (Derivative Financial Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Derivative [Line Items]
Cash held as collateral	$ 216		$ 216		$ 1,451
Foreign currency derivative contracts
Derivative [Line Items]
Notional amount of foreign currency derivative contracts	0		0		2,866
Fair value of foreign currency derivatives contracts	0		0		$ 63
Foreign currency derivative contracts | Other (income) expense
Derivative [Line Items]
Gain (losses) on Derivative Instruments Recognized in Statements of Operations	$ 0	$ 38	$ (50)	$ 213