Computation of Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2015	Sep. 27, 2014	Sep. 26, 2015	Sep. 27, 2014
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
As reported - basic	$ 38,624	$ 37,926	$ 82,299	$ 71,669
Net income per common share - diluted	13,507	13,495	13,602	13,472
As reported - basic	$ 2.93	$ 2.91	$ 6.20	$ 5.50
Net income per common share - diluted	$ 2.85	$ 2.79	$ 6.02	$ 5.29
Common Class A
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
As reported - basic	$ 28,286	$ 26,878	$ 59,963	$ 50,348
Class B Common Stock	10,159	10,833	21,951	20,919
Net effect of unvested participating shares	5	7	11	16
Net income per common share - diluted	$ 38,450	$ 37,718	$ 81,925	$ 71,283
As reported - basic	9,655	9,248	9,667	9,148
Add: effect of dilutive potential common share-based awards	385	520	396	523
Class B Common Stock	3,467	3,727	3,539	3,801
Net income per common share - diluted	13,507	13,495	13,602	13,472
As reported - basic	$ 2.93	$ 2.91	$ 6.20	$ 5.50
Net income per common share - diluted	$ 2.85	$ 2.79	$ 6.02	$ 5.29