Derivatives - Fair value of derivative instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Interest rate swap contract | Derivatives Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Derivative liabilities	$ 372	$ (38)
Interest rate swap contract | Derivatives Designated as Hedging Instruments | Prepaids and other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	134	0
Interest rate swap contract | Derivatives Designated as Hedging Instruments | Other assets
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	238	0
Interest rate swap contract | Derivatives Designated as Hedging Instruments | Accrued liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	0	(38)
Foreign currency forward contracts | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	390	0
Foreign currency forward contracts | Not Designated as Hedging Instrument | Prepaids and other current assets
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset	$ 390	$ 0