Financial instruments - Fair values of derivative instruments designated as hedging instruments (Details) - USD ($) $ in Thousands	Jul. 01, 2018	Dec. 31, 2017
Derivatives Fair Value [Line Items]
Total asset derivatives	$ 1,954	$ 1,221
Total liability derivatives	861	1,426
Foreign exchange contract | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	1,954	1,221
Foreign exchange contract | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	861	1,426
Foreign exchange contract | Designated as Hedging Instrument | Cash flow hedging | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	1,513	914
Foreign exchange contract | Designated as Hedging Instrument | Cash flow hedging | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	759	1,373
Foreign exchange contract | Not Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	441	307
Foreign exchange contract | Not Designated as Hedging Instrument | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	$ 102	$ 53