Fair value measurement (Details) (USD $) In Thousands, unless otherwise specified	Jun. 24, 2012	Dec. 25, 2011
assets and liabilities measured at fair value
Employee compensation related investments	$ 30,299	$ 17,224
Rabbi trust investments	27,345	26,162
Total assets	57,644	43,386
Contingent consideration payable	21,925	15,808
Total liabilities	21,925	15,808
Level 1[Member]
assets and liabilities measured at fair value
Employee compensation related investments	30,299	17,224
Rabbi trust investments	27,345	26,162
Total assets	57,644	43,386
Contingent consideration payable
Total liabilities
Level 2 [Member]
assets and liabilities measured at fair value
Employee compensation related investments
Rabbi trust investments
Total assets
Contingent consideration payable
Total liabilities
Level 3 [Member]
assets and liabilities measured at fair value
Employee compensation related investments
Rabbi trust investments
Total assets
Contingent consideration payable	21,925	15,808
Total liabilities	$ 21,925	$ 15,808