Derivative financial instruments and fair value measurements (Contingent Consideration) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Apr. 05, 2014	Mar. 30, 2013	Apr. 05, 2014 Fair Value, Inputs, Level 3 [Member]	Dec. 28, 2013 Fair Value, Inputs, Level 3 [Member]
Derivative [Line Items]
Business Acquisition, Contingent Consideration, at Fair Value			$ (545)	$ (2,671)
Fair value of contingent consideration	(1,373)	0
Business Acquisition, Contingent Consideration, at Fair Value			$ (545)	$ (2,671)