Financial instruments - Fair values of derivative instruments designated as hedging instruments (Details) - USD ($) $ in Thousands	Oct. 01, 2017	Dec. 31, 2016
Derivatives Fair Value [Line Items]
Total asset derivatives	$ 2,002	$ 1,157
Total liability derivatives	330	2,257
Foreign exchange contract | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	2,002	1,157
Foreign exchange contract | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	330	2,257
Foreign exchange contract | Designated as Hedging Instrument | Cash flow hedging | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	1,684	667
Foreign exchange contract | Designated as Hedging Instrument | Cash flow hedging | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	237	2,139
Foreign exchange contract | Not Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	318	490
Foreign exchange contract | Not Designated as Hedging Instrument | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	$ 93	$ 118