Financial instruments - Fair values of derivative instruments designated as hedging instruments (Details) - USD ($) $ in Thousands	Sep. 25, 2016	Dec. 31, 2015
Derivatives Fair Value [Line Items]
Total asset derivatives	$ 1,134	$ 329
Total liability derivatives	2,403	1,298
Forward Contracts | Foreign Currency Contract | Prepaid Expenses and Other Current Assets
Derivatives Fair Value [Line Items]
Total asset derivatives	1,134	329
Forward Contracts | Foreign Currency Contract | Other Current Liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	2,403	1,298
Forward Contracts | Designated as Hedging Instrument | Cash flow hedging | Foreign Currency Contract | Prepaid Expenses and Other Current Assets
Derivatives Fair Value [Line Items]
Total asset derivatives	615	285
Forward Contracts | Designated as Hedging Instrument | Cash flow hedging | Foreign Currency Contract | Other Current Liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	2,369	807
Forward Contracts | Not Designated as Hedging Instrument | Foreign Currency Contract | Prepaid Expenses and Other Current Assets
Derivatives Fair Value [Line Items]
Total asset derivatives	519	44
Forward Contracts | Not Designated as Hedging Instrument | Foreign Currency Contract | Other Current Liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	$ 34	$ 491