Derivative Financial Instruments - Narratives (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 instrument	Sep. 30, 2012 USD ($) instrument	Jun. 24, 2012 USD ($) instrument
Derivative
Unrealized Gain (Loss) on Foreign Currency Derivatives, Net, before Tax	$ 867	$ 0
Foreign exchange derivative
Derivative
Derivative Asset	864
Derivative Liability	0
Cash Flow Hedge | Designated As Hedging | Foreign currency forward contracts
Derivative
Foreign currency cash flow hedge gain to be reclassified during next 12 months	644
Derivative, notional amount	19,200
Derivative Asset	864
Derivative Liability			$ (742)	$ (601)
Cash Flow Hedge | Designated As Hedging | Interest rate swap
Derivative
Derivative, notional amount						$ 25,000	$ 50,000
Number of derivative instruments held | instrument					3	4	4
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months	616
Derivative Asset	$ 1,358		$ 971	$ 753