Financial instruments - After tax gains and losses reclassified from accumulated other comprehensive income into income (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 25, 2016	Sep. 27, 2015	Sep. 25, 2016	Sep. 27, 2015
Forward Contracts \| Cash flow hedging \| Foreign Currency Contract
Derivative Instruments, Gain (Loss) [Line Items]
After Tax Gain (Loss) Recognized in OCI	$ (223)	$ (730)	$ 761	$ (1,489)

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration