Financial instruments - After tax (gain)/loss reclassified from accumulated other comprehensive income into income (Details) - Net proceeds from share based compensation plans and the related tax impacts - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 26, 2016	Jun. 28, 2015	Jun. 26, 2016	Jun. 28, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives qualifying as hedges, net of tax	$ (496)	$ (803)
Foreign Currency Contract
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives qualifying as hedges, net of tax	$ (496)	$ (803)
Forward Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives qualifying as hedges, net of tax			$ 984	$ (759)
Forward Contracts | Foreign Currency Contract
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives qualifying as hedges, net of tax			$ 984	$ (759)