Computation of Earnings Per Share, Basic (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 27, 2015	Jun. 28, 2014	Jun. 27, 2015	Jun. 28, 2014
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net Income		$ 29,932	$ 25,428	$ 43,675	$ 33,743
Allocation of net income for basic:
Allocation of net income for basic common stock		29,932	25,428	43,675	33,743
Allocation of net income for basic unvested participating shares		$ 139	$ 140	$ 206	$ 188
Weighted average number of shares for basic:
Weighted average number of shares for basic unvested participating shares		62	72	63	73
Shares used in net income per common share - basic		13,342	13,038	13,311	13,007
Net income per share for basic:
Net Income Per Common Stock		$ 2.24	$ 1.95	$ 3.28	$ 2.59
Common Class A
Allocation of net income for basic:
Allocation of net income for basic common stock		$ 21,870	$ 17,935	$ 31,740	$ 23,600
Weighted average number of shares for basic:
Weighted average number of shares for basic common stock		9,748	9,196	9,673	9,097
Net income per share for basic:
Net Income Per Common Stock		$ 2.24	$ 1.95	$ 3.28	$ 2.59
Common Class B
Allocation of net income for basic:
Allocation of net income for basic common stock		$ 7,923	$ 7,353	$ 11,729	$ 9,955
Weighted average number of shares for basic:
Weighted average number of shares for basic common stock	[1]	3,532	3,770	3,575	3,837
Net income per share for basic:
Net Income Per Common Stock		$ 2.24	$ 1.95	$ 3.28	$ 2.59

[1] Change in Class B Common Stock resulted from the conversion of 150,000 shares to Class A Common Stock on May 6, 2015, with the 26-week number of shares reflecting the weighted average for the period.