Fair Value Measurements - Reconciliation of Recurring Financial Liability Related to Interest Rate Swaps (Details) - Designated As Hedging - Cash Flow Hedge - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Interest rate swap
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Cost of Hedge Net of Cash Received	$ 0
Reconciliation of the recurring financial derivatives
Beginning balance, derivative asset	971	$ 753
Total gains/(losses) for the period:
Included in earnings	(284)	0
Included in other comprehensive income	671	218
Ending balance, derivative asset	1,358	971
Foreign currency hedges
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Cost of Hedge Net of Cash Received	(143)	(132)
Reconciliation of the recurring financial derivatives
Beginning balance, derivative liability	742	601
Total gains/(losses) for the period:
Included in earnings	200	38
Included in other comprehensive income	1,549	(47)
Ending balance, derivative liability		$ 742
Ending balance, derivative asset	$ 864