Market Risks and Derivative Hedge Contracts (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Derivatives, Fair Value [Line Items]
Foreign Currency Derivative Instruments Not Designated as Hedging Instruments, Liability at Fair Value	$ (161)		$ (127)
Derivative [Line Items]
Net gains associated with foreign currency derivatives	0	$ 700
Forward Purchase Contract, Euro [Member]
Derivative [Line Items]
U.S. Dollar notional amount	$ 1,117
Traded exchange rate	1.24
Value date	Apr. 18, 2018
Forward Purchase Contract, Pounds Sterling [Member]
Derivative [Line Items]
U.S. Dollar notional amount	$ 5,624
Traded exchange rate	1.41
Value date	Apr. 18, 2018
Forward Purchase Contract, Brazil Real [Member]
Derivative [Line Items]
U.S. Dollar notional amount	$ 5,925
Traded exchange rate	1.30
Value date	Apr. 18, 2018
Forward Purchase Contract, Mexican Pesos [Member]
Derivative [Line Items]
U.S. Dollar notional amount	$ 1,653
Traded exchange rate	18.76
Value date	Apr. 18, 2018
Forward Sale Contract, Norwegian Krone [Member]
Derivative [Line Items]
U.S. Dollar notional amount	$ 3,782
Traded exchange rate	7.75
Value date	Apr. 18, 2018
Forward Sale Contract, Mexican Pesos (2) [Member]
Derivative [Line Items]
U.S. Dollar notional amount	$ 6,760
Traded exchange rate	18.79
Value date	Apr. 18, 2018
Current Assets [Member]
Derivatives, Fair Value [Line Items]
Forward purchase contracts	$ 56			$ 130
Forward sale contracts	48		111
Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Forward purchase contracts	(6)		(113)
Forward sale contracts	$ (259)		$ (255)