Fair value measurement (Details) (USD $) In Thousands, unless otherwise specified									9 Months Ended
	Sep. 29, 2013	Dec. 30, 2012	Sep. 29, 2013 Level 1 [Member]	Dec. 30, 2012 Level 1 [Member]	Sep. 29, 2013 Level 2 [Member]	Dec. 30, 2012 Level 2 [Member]	Sep. 29, 2013 Level 3 [Member]	Dec. 30, 2012 Level 3 [Member]	Sep. 29, 2013 Minimum [Member] Level 3 [Member]	Sep. 29, 2013 Maximum [Member] Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Discount Rate									10.00%	30.00%
assets and liabilities measured at fair value
Employee compensation related investments	$ 27,105	$ 23,043	$ 27,105	$ 23,043	$ 0	$ 0	$ 0	$ 0
Sundry investments	32,466	29,090	32,466	29,090	0	0	0	0
Total assets	59,571	52,133	59,571	52,133	0	0	0	0
Contingent consideration payable	33,141	26,170	0	0	0	0	33,141	26,170
Total liabilities	$ 33,141	$ 26,170	$ 0	$ 0	$ 0	$ 0	$ 33,141	$ 26,170