Financial instruments Fair values of derivative instruments designated as hedging instruments (Details) - USD ($) $ in Thousands	Jun. 28, 2015	Dec. 31, 2014
Derivatives Fair Value [Line Items]
Total asset derivatives	$ 419	$ 0
Total liability derivatives	1,808	0
Foreign Currency Forward Contract | Prepaid Expenses and Other Current Assets
Derivatives Fair Value [Line Items]
Total asset derivatives	419	0
Foreign Currency Forward Contract | Other Current Liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	$ 1,808	$ 0