Selected Quarterly Financial and Earnings Data (Unaudited) Quarterly Table (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares	Oct. 01, 2017 USD ($) $ / shares	Jul. 02, 2017 USD ($) $ / shares	Apr. 02, 2017 USD ($) $ / shares	Oct. 02, 2016 USD ($) $ / shares	Jul. 03, 2016 USD ($) $ / shares	Apr. 03, 2016 USD ($) $ / shares	Jan. 03, 2016 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Jan. 01, 2017 USD ($) $ / shares	Jan. 03, 2016 USD ($) $ / shares
Quarterly Financial Information Disclosure [Abstract]
Restaurant sales	$ 283,967	$ 285,235	$ 279,478	$ 239,852	$ 238,870	$ 241,368	$ 222,519	$ 240,826	$ 1,088,532	$ 943,583	$ 859,004
Operating income (loss)	9,860	8,684	12,335	(1,406)	9,049	13,896	6,680	6,077	29,473	35,702	31,208
Net income (loss)	$ 3,921	$ 2,795	$ 6,039	$ (5,596)	$ 4,489	$ 9,376	$ 2,145	$ 29,462	$ 7,159	$ 45,472	$ 4
Basic and diluted net income (loss) per share | $ / shares	$ 0.09	$ 0.06	$ 0.13	$ (0.16)	$ 0.10	$ 0.21	$ 0.05	$ 0.65	$ 0.16	$ 1.01	$ 0.00
Restaurants at end of period	807	798	799	788	734	723	717		807	753