Fair value measurement (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 30, 2012
assets and liabilities measured at fair value
Employee compensation related investments	$ 25,747	$ 23,043
Sundry investments	30,933	29,090
Total assets	56,680	52,133
Contingent consideration payable	30,384	26,170
Total liabilities	30,384	26,170
Level 1 [Member]
assets and liabilities measured at fair value
Employee compensation related investments	25,747	23,043
Sundry investments	30,933	29,090
Total assets	56,680	52,133
Contingent consideration payable	0	0
Total liabilities	0	0
Level 2 [Member]
assets and liabilities measured at fair value
Employee compensation related investments	0	0
Sundry investments	0	0
Total assets	0	0
Contingent consideration payable	0	0
Total liabilities	0	0
Level 3 [Member]
assets and liabilities measured at fair value
Employee compensation related investments	0	0
Sundry investments	0	0
Total assets	0	0
Contingent consideration payable	30,384	26,170
Total liabilities	$ 30,384	$ 26,170