Income taxes (Unrecognized tax benefits) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 30, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Unrecognized Tax Benefits	$ 452	$ 1,720
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions	(452)	(1,268)
Unrecognized Tax Benefits	$ 0	$ 452

Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319

Name:	us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration