Earnings per share (Narrative) (Details) - shares	12 Months Ended
	Dec. 30, 2017	Dec. 31, 2016	Jan. 02, 2016
Earnings Per Share Abstract
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	2,540,189	2,321,448	856,492
Dilutive Securities, Effect on Basic Earnings Per Share, Including Options and Restrictive Stock Units	0	0	0