Computation of Earnings Per Share, Basic (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended
		Apr. 01, 2017	Mar. 26, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income		$ 5,711	$ 7,032
Allocation of net income for basic:
Allocation of net income for basic common stock		5,711	7,032
Allocation of net income for basic unvested participating shares		$ 32	$ 34
Weighted average number of shares for basic:
Weighted average number of shares for basic unvested participating shares		71	62
Shares used in net income per common share - basic		12,471	12,804
Net income per share for basic:
Net income per common share - basic		$ 0.46	$ 0.55
Common Class A
Allocation of net income for basic:
Allocation of net income for basic common stock		$ 4,227	$ 5,149
Weighted average number of shares for basic:
Weighted-average number of common shares - basic		9,230	9,375
Net income per share for basic:
Net income per common share - basic		$ 0.46	$ 0.55
Common Class B
Allocation of net income for basic:
Allocation of net income for basic common stock		$ 1,452	$ 1,849
Weighted average number of shares for basic:
Weighted-average number of common shares - basic	[1]	3,170	3,367
Net income per share for basic:
Net income per common share - basic		$ 0.46	$ 0.55

[1] Change in Class B Common Stock resulted from the conversion of 100,000 shares to Class A Common Stock on March 7, 2017, 45,000 shares to Class A Common Stock on November 30, 2016 and 125,000 shares to Class A Common Stock on November 4, 2016, with the ending number of shares reflecting the weighted average for the periods.