Financial instruments - Fair value of derivatives (Details) - Cash Flow Hedging - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Derivatives Fair Value [Line Items]
Asset derivatives	$ 1,157	$ 329
Liability derivatives	2,257	1,298
Foreign Currency Exchange Contracts | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Asset derivatives	1,157	329
Foreign Currency Exchange Contracts | Other current liabilities
Derivatives Fair Value [Line Items]
Liability derivatives	2,257	1,298
Foreign Currency Exchange Contracts | Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Asset derivatives	667	285
Foreign Currency Exchange Contracts | Designated as Hedging Instrument | Other current liabilities
Derivatives Fair Value [Line Items]
Liability derivatives	2,139	807
Foreign Currency Exchange Contracts | Not Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Asset derivatives	490	44
Foreign Currency Exchange Contracts | Not Designated as Hedging Instrument | Other current liabilities
Derivatives Fair Value [Line Items]
Liability derivatives	$ 118	$ 491