Earnings per share (Narrative) (Details) - shares	12 Months Ended
Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016
Earnings Per Share Abstract
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	2,384,249	2,540,189	2,321,448
Dilutive Securities, Effect on Basic Earnings Per Share, Including Options and Restrictive Stock Units	0	0	0

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Name:	us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration