Computation of Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Earning to common shareholders, As reported - basic	$ 38,007	$ 33,683	$ 70,852	$ 68,519
Earnings to Common Shareholders, Class B Common Stock	9,800	8,730	18,169	17,493
Earnings to Common Shareholders, Net effect of unvested participating shares	5	2	5	4
Earnings to Common Shareholders, Net income per common share-diluted	$ 37,591	$ 33,504	$ 70,225	$ 68,144
Weighted-average number of common shares-basic	11,705	11,951	11,769	12,226
Add: effect of dilutive potential common shares Share-based awards	127	151	109	140
Add: effect of dilutive potential common shares, Class B Common Stock	3,018	3,097	3,018	3,122
Weighted-average number of common shares-diluted	11,702	12,037	11,773	12,299
Net income per common share-basic	$ 3.25	$ 2.82	$ 6.02	$ 5.60
Net income per common share-diluted	$ 3.21	$ 2.78	$ 5.96	$ 5.54
Common Class A
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Earning to common shareholders, As reported - basic	$ 27,786	$ 24,772	$ 52,051	$ 50,647
Weighted-average number of common shares-basic	8,557	8,789	8,646	9,037
Net income per common share-basic	$ 3.25	$ 2.82	$ 6.02	$ 5.60