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, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest rate swap
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Cost of Hedge Net of Cash Received	$ (6)
Reconciliation of the recurring financial derivatives
Beginning balance, derivative liability	753	$ (768)
Total gains/(losses) for the period:
Included in earnings	(6)		$ 928
Included in other comprehensive income	(91)		593
Ending balance, derivative liability	650	753	(768)
Foreign currency hedges
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Cost of Hedge Net of Cash Received	(75)	54
Reconciliation of the recurring financial derivatives
Beginning balance, derivative liability	(601)	0
Total gains/(losses) for the period:
Included in earnings	(51)	(18)
Included in other comprehensive income	987	(637)
Ending balance, derivative liability		$ (601)	$ 0
Ending balance, derivative asset	$ 260