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	Mar. 27, 2016	Dec. 31, 2015
Derivatives Fair Value [Line Items]
Total asset derivatives	$ 2,511	$ 329
Total liability derivatives	1,873	1,298
Foreign Currency Contract | Prepaid Expenses and Other Current Assets
Derivatives Fair Value [Line Items]
Total asset derivatives	2,511	329
Foreign Currency Contract | Other Current Liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	1,873	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	2,252	285
Forward Contracts | Designated as Hedging Instrument | Foreign Currency Contract | Other Current Liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	1,705	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	259	44
Forward Contracts | Not Designated as Hedging Instrument | Foreign Currency Contract | Other Current Liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	$ 168	$ 491