Goodwill and intangible assets (Summary of Changes - Intangible Assets) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 30, 2017	Dec. 31, 2016	Jan. 02, 2016
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Net, Opening	$ 183,524	$ 195,008
Acquisitions	0	0
Impairments	(456)
Intangible asset amortization	(11,195)	(11,282)	$ (4,951)
Finite Lived Intangible Assets Foreign Currency Translation Gain Loss	186	(202)
Finite-Lived Intangible Assets, Net, Ending	172,059	183,524	195,008
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Net, Opening	183,258	194,633
Acquisitions	0	0
Impairments	(456)
Intangible asset amortization	(11,086)	(11,166)
Finite Lived Intangible Assets Foreign Currency Translation Gain Loss	186	(209)
Finite-Lived Intangible Assets, Net, Ending	171,902	183,258	194,633
Patents, Trademarks and Other
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Net, Opening	266	375
Acquisitions	0	0
Impairments	0
Intangible asset amortization	(109)	(116)
Finite Lived Intangible Assets Foreign Currency Translation Gain Loss	0	7
Finite-Lived Intangible Assets, Net, Ending	$ 157	$ 266	$ 375

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
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-Section 45
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-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
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-Section 50
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Amount of foreign currency translation gain (loss) which increases (decreases) assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, from an acquisition.

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