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Income taxes (Details)
3 Months Ended
Mar. 26, 2013
Mar. 27, 2012
Reconciliation of the statutory federal income tax rate to the entity's effective tax rate    
Tax at statutory federal rate (as a percent) 35.00% 35.00%
State and local tax, net of federal benefit (as a percent) 3.70% 3.80%
FICA tip tax credit (as a percent) (5.20%) (5.70%)
Work opportunity tax credit (as a percent) (3.50%) (0.80%)
Incentive stock options (as a percent) (0.60%) (0.30%)
Nondeductible officer compensation (as a percent) 0.70% 1.00%
Other (as a percent) (1.40%) (0.50%)
Total (as a percent) 28.70% 32.50%