Financial instruments - Fair value of derivatives (Details) - Cash Flow Hedging - Designated as Hedging Instrument - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Derivatives Fair Value [Line Items]
Asset derivatives	$ 285	$ 0
Liability derivatives	807	0
Foreign Currency Exchange Contracts | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Asset derivatives	285	0
Foreign Currency Exchange Contracts | Other current liabilities
Derivatives Fair Value [Line Items]
Liability derivatives	$ 807	$ 0