Computation of Earnings Per Share, Basic (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, Basic, by Common Class, Including Two Class Method [Line Items]
Net Income		$ 38,624	$ 37,926	$ 82,299	$ 71,669
Allocation of net income for basic:
Allocation of net income for basic common stock		38,624	37,926	82,299	71,669
Allocation of net income for basic unvested participating shares		$ 179	$ 215	$ 385	$ 402
Weighted average number of shares for basic:
Weighted average number of shares for basic unvested participating shares		61	74	62	73
Shares used in net income per common share - basic		13,183	13,049	13,268	13,022
Net income per share for basic:
Net Income Per Common Stock		$ 2.93	$ 2.91	$ 6.20	$ 5.50
Common Class A
Allocation of net income for basic:
Allocation of net income for basic common stock		$ 28,286	$ 26,878	$ 59,963	$ 50,348
Weighted average number of shares for basic:
Weighted average number of shares for basic common stock		9,655	9,248	9,667	9,148
Net income per share for basic:
Net Income Per Common Stock		$ 2.93	$ 2.91	$ 6.20	$ 5.50
Common Class B
Allocation of net income for basic:
Allocation of net income for basic common stock		$ 10,159	$ 10,833	$ 21,951	$ 20,919
Weighted average number of shares for basic:
Weighted average number of shares for basic common stock	[1]	3,467	3,727	3,539	3,801
Net income per share for basic:
Net Income Per Common Stock		$ 2.93	$ 2.91	$ 6.20	$ 5.50

[1] Change in Class B Common Stock resulted from the conversion of 110,000 share to Class A Common Stock on November 4, 2014 and 150,000 shares to Class A Common Stock on May 6, 2015, with the thirteen and thirty-nine-week number of shares reflecting the weighted average for the periods.