Fair Value Measurements - Summary of Financial Liability Measured at Fair Value on a Recurring Basis (Details) - Designated As Hedging - Cash Flow Hedge - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Interest rate swap
Recurring financial liability that was measured at carrying value
Realized gain (loss) on cash flow hedge	$ (284)	$ 0
Derivative Asset	1,358	971	$ 753
Foreign currency hedges
Recurring financial liability that was measured at carrying value
Derivative liability		742	$ 601
Realized gain (loss) on cash flow hedge	200	38
Derivative Asset	864
Recurring | Significant Other Observable Inputs (Level 2) | Interest rate swap
Recurring financial liability that was measured at carrying value
Derivative liability	(1,358)	(971)
Recurring | Significant Other Observable Inputs (Level 2) | Foreign currency hedges
Recurring financial liability that was measured at carrying value
Derivative liability		742
Derivative Asset	864
Recurring | Carrying Value | Interest rate swap
Recurring financial liability that was measured at carrying value
Derivative liability	(1,358)	(971)
Recurring | Carrying Value | Foreign currency hedges
Recurring financial liability that was measured at carrying value
Derivative liability		$ 742
Derivative Asset	$ 864