Financial instruments - Fair values of derivative instruments designated as hedging instruments (Details) - Net proceeds from share based compensation plans and the related tax impacts - USD ($) $ in Thousands	Jun. 26, 2016	Dec. 31, 2015
Derivatives Fair Value [Line Items]
Total asset derivatives	$ 1,666	$ 329
Total liability derivatives	4,772	1,298
Forward Contracts | Foreign Currency Contract | Prepaid Expenses and Other Current Assets
Derivatives Fair Value [Line Items]
Total asset derivatives	1,666	329
Forward Contracts | Foreign Currency Contract | Other Current Liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	4,772	1,298
Forward Contracts | Designated as Hedging Instrument | Foreign Currency Contract | Prepaid Expenses and Other Current Assets
Derivatives Fair Value [Line Items]
Total asset derivatives	1,211	285
Forward Contracts | Designated as Hedging Instrument | Foreign Currency Contract | Other Current Liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	3,366	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	455	44
Forward Contracts | Not Designated as Hedging Instrument | Foreign Currency Contract | Other Current Liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	$ 1,406	$ 491