Financial instruments - Fair values of derivative instruments designated as hedging instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivatives Fair Value [Line Items]
Total asset derivatives	$ 2,333	$ 1,221
Total liability derivatives	1,636	1,426
Foreign exchange contract | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	2,333	1,221
Foreign exchange contract | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	1,636	1,426
Foreign exchange contract | Designated as Hedging Instrument | Cash flow hedging | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	2,158	914
Foreign exchange contract | Designated as Hedging Instrument | Cash flow hedging | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	572	1,373
Foreign exchange contract | Not Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	175	307
Foreign exchange contract | Not Designated as Hedging Instrument | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	$ 1,064	$ 53