Financial instruments Fair values of derivative instruments designated as hedging instruments (Details) - Designated as Hedging Instrument - Cash Flow Hedging - USD ($) $ in Thousands	Sep. 27, 2015	Dec. 31, 2014
Derivatives Fair Value [Line Items]
Total asset derivatives	$ 834	$ 0
Total liability derivatives	3,247	0
Foreign Currency Forward Contract | Prepaid Expenses and Other Current Assets
Derivatives Fair Value [Line Items]
Total asset derivatives	834	0
Foreign Currency Forward Contract | Other Current Liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	$ 3,247	$ 0