Computation of Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended
		Mar. 31, 2018	Apr. 01, 2017
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Allocation of net income for basic common stock		$ 9,310	$ 5,711
Earnings to Common Shareholders, Net effect of unvested participating shares		1
Earnings to Common Shareholders, Net income per common share - diluted		$ 9,253	$ 5,679
Weighted-average number of common shares - basic		11,805	12,471
Add: effect of dilutive potential common shares Share-based awards		99	116
Weighted-average number of common shares - diluted		11,831	12,516
Net income per common share-basic		$ 0.79	$ 0.46
Net income per common share-diluted		$ 0.78	$ 0.45
Common Class A
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Allocation of net income for basic common stock		$ 6,872	$ 4,227
Weighted-average number of common shares - basic		8,714	9,230
Net income per common share-basic		$ 0.79	$ 0.46
Common Class B
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Allocation of net income for basic common stock		$ 2,380	$ 1,452
Earnings to Common Shareholders, Class B Common Stock		$ 2,380	$ 1,452
Weighted-average number of common shares - basic	[1]	3,018	3,170
Add: effect of dilutive potential common shares, Class B Common Stock		3,018	3,170
Net income per common share-basic		$ 0.79	$ 0.46

[1] Change in Class B Common Stock resulted from the conversion of 79,000 shares to Class A Common Stock on October 31, 2017 with the ending number of shares reflecting the weighted average for the period.