Computation of Earnings Per Share, Basic (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 24, 2016	Sep. 30, 2017	Sep. 24, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income		$ 33,683	$ 31,530	$ 68,519	$ 65,183
Allocation of net income for basic:
Allocation of net income for basic common stock		33,683	31,530	68,519	65,183
Allocation of net income for basic unvested participating shares		$ 181	$ 160	$ 379	$ 328
Weighted average number of shares for basic:
Weighted average number of shares for basic unvested participating shares		65	63	67	64
Shares used in net income per common share - basic		11,951	12,448	12,226	12,622
Net income per share for basic:
Net income per common share - basic		$ 2.82	$ 2.53	$ 5.60	$ 5.16
Common Class A
Allocation of net income for basic:
Allocation of net income for basic common stock		$ 24,772	$ 22,841	$ 50,647	$ 47,465
Weighted average number of shares for basic:
Weighted-average number of common shares - basic		8,789	9,018	9,037	9,191
Net income per share for basic:
Net income per common share - basic		$ 2.82	$ 2.53	$ 5.60	$ 5.16
Common Class B
Allocation of net income for basic:
Allocation of net income for basic common stock		$ 8,730	$ 8,529	$ 17,493	$ 17,390
Weighted average number of shares for basic:
Weighted-average number of common shares - basic	[1]	3,097	3,367	3,122	3,367
Net income per share for basic:
Net income per common share - basic		$ 2.82	$ 2.53	$ 5.60	$ 5.16

[1] Change in Class B Common Stock resulted from the conversion of 100,000 shares to Class A Common Stock on March 7, 2017, 45,000 shares to Class A Common Stock on November 30, 2016 and 125,000 shares to Class A Common Stock on November 4, 2016, with the ending number of shares reflecting the weighted average for the periods.