Computation of Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended
		Apr. 01, 2017	Mar. 26, 2016
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Allocation of net income for basic common stock		$ 5,711	$ 7,032
Earnings to Common Shareholders, Net effect of unvested participating shares		0	1
Earnings to Common Shareholders, Net income per common share - diluted		$ 5,679	$ 6,999
Add: effect of dilutive potential common, Share-based awards		116	346
Weighted-average number of common shares - diluted		12,516	13,088
Net income per common share-basic		$ 0.46	$ 0.55
Net income per common share-diluted		$ 0.45	$ 0.53
Common Class A
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Allocation of net income for basic common stock		$ 4,227	$ 5,149
Weighted-average number of common shares - basic		9,230	9,375
Net income per common share-basic		$ 0.46	$ 0.55
Common Class B
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Allocation of net income for basic common stock		$ 1,452	$ 1,849
Earnings to Common Shareholders, Class B Common Stock		$ 1,452	$ 1,849
Weighted-average number of common shares - basic	[1]	3,170	3,367
Add: effect of dilutive potential common, Class B Common Stock		3,170	3,367
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.