Income taxes (Details)	3 Months Ended		9 Months Ended
	Sep. 24, 2013	Sep. 25, 2012	Sep. 24, 2013	Sep. 25, 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%	35.00%	35.00%
State and local tax, net of federal benefit (as a percent)	3.70%	3.80%	3.70%	3.80%
FICA tip tax credit (as a percent)	(7.50%)	(6.40%)	(6.30%)	(5.80%)
Work opportunity tax credit (as a percent)	(1.10%)	(0.90%)	(2.20%)	(0.80%)
Incentive stock options (as a percent)	(1.20%)	(0.20%)	(0.80%)	(0.30%)
Nondeductible officer compensation (as a percent)	0.50%	1.40%	0.40%	1.10%
Other (as a percent)	1.00%		(0.20%)	(0.30%)
Total (as a percent)	30.40%	32.70%	29.60%	32.70%