Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Preferred Shares	Ordinary Shares	Additional Paid-in Capital	Retained Earnings (Accumulated Deficit)
Balance at Dec. 26, 2014	$ 90,061	$ 142,728		$ 1,886	$ (54,553)
Balance, Shares at Dec. 26, 2014		17,722,808	22,377
Share-based compensation expense	1,118			1,118
Ordinary shares issued from vesting of restricted share units, Shares			43,032
Dividend to shareholders	(22,127)				(22,127)
Net income	5,626				5,626
Balance at Dec. 25, 2015	74,678	$ 142,728		3,004	(71,054)
Balance, Shares at Dec. 25, 2015		17,722,808	65,409
Ordinary shares issued from initial public offering, net of transaction costs	47,103			47,103
Ordinary shares issued from initial public offering, net of transaction costs, shares			5,877,778
Conversion of preferred shares to ordinary shares	0	$ (142,728)	$ 2	142,726
Conversion of preferred shares to ordinary shares, shares		(17,722,808)	17,722,808
Share-based compensation expense	$ 3,216			3,216
Ordinary shares issued from vesting of restricted share units, Shares	0		191,386
Net income	$ 16,662				16,662
Balance at Dec. 30, 2016	141,659		$ 2	196,049	(54,392)
Balance, Shares at Dec. 30, 2016			23,857,381
Ordinary shares issued from initial public offering, net of transaction costs	7,278		$ 1	7,277
Ordinary shares issued from initial public offering, net of transaction costs, shares			881,667
Ordinary shares issued from exercise of stock options	9,141			9,141
Ordinary shares issued from exercise of stock options, shares			1,078,182
Share-based compensation expense	2,230			2,230
Ordinary shares issued from vesting of restricted share units, Shares			74,932
Net income	56,454				56,454
Balance at Dec. 29, 2017	$ 216,762		$ 3	$ 214,697	$ 2,062
Balance, Shares at Dec. 29, 2017			25,892,162