Financial instruments Fair values of derivative instruments designated as hedging instruments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 28, 2014	Dec. 31, 2013
Derivatives Fair Value [Line Items]
Total asset derivatives	$ 959	$ 0
Total liability derivatives	471	0
Foreign Currency Forward Contract | Prepaid Expenses and Other Current Assets
Derivatives Fair Value [Line Items]
Total asset derivatives	959	0
Foreign Currency Forward Contract | Other Current Liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	$ 471	$ 0