Selected Quarterly Financial and Earnings Data (Unaudited) Quarterly Table (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 30, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jul. 01, 2018 USD ($) $ / shares	Apr. 01, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Oct. 01, 2017 USD ($) $ / shares	Jul. 02, 2017 USD ($) $ / shares	Apr. 02, 2017 USD ($) $ / shares	Dec. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2017 USD ($)
Quarterly Financial Information Disclosure [Abstract]
Restaurant sales	$ 307,754	$ 296,917	$ 303,050	$ 271,586	$ 283,967	$ 285,235	$ 279,478	$ 239,852	$ 1,179,307	$ 1,088,532	$ 943,583
Operating income (loss)	7,190	9,668	13,833	2,664	9,860	8,684	12,335	(1,406)	33,355	29,473	35,702
Net income (loss)	$ 1,807	$ 3,611	$ 7,788	$ (3,102)	$ 3,921	$ 2,795	$ 6,039	$ (5,596)	$ 10,104	$ 7,159	$ 45,472
Basic and diluted net income (loss) per share | $ / shares	$ 0.04	$ 0.08	$ 0.17	$ (0.09)	$ 0.09	$ 0.06	$ 0.13	$ (0.16)
Restaurants at end of period	849	838	807	807	807	798	799	788	849	807