Financial instruments - Fair value of derivatives (Details) - Cash Flow Hedging - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivatives Fair Value [Line Items]
Asset derivatives:	$ 1,221	$ 1,157
Liability derivatives:	1,426	2,257
Foreign Currency Exchange Contracts | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Asset derivatives:	1,221	1,157
Foreign Currency Exchange Contracts | Other current liabilities
Derivatives Fair Value [Line Items]
Liability derivatives:	1,426	2,257
Foreign Currency Exchange Contracts | Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Asset derivatives:	914	667
Foreign Currency Exchange Contracts | Designated as Hedging Instrument | Other current liabilities
Derivatives Fair Value [Line Items]
Liability derivatives:	1,373	2,139
Foreign Currency Exchange Contracts | Not Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Asset derivatives:	307	490
Foreign Currency Exchange Contracts | Not Designated as Hedging Instrument | Other current liabilities
Derivatives Fair Value [Line Items]
Liability derivatives:	$ 53	$ 118