Computation of Earnings Per Share, Basic (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended
		Mar. 31, 2018	Apr. 01, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income		$ 9,310	$ 5,711
Allocation of net income for basic:
Allocation of net income for basic common stock		9,310	5,711
Allocation of net income for basic unvested participating shares		$ 58	$ 32
Weighted average number of shares for basic:
Weighted-average number of common shares - basic		11,805	12,471
Net income per share for basic:
Net income per common share - basic		$ 0.79	$ 0.46
Common Class A
Allocation of net income for basic:
Allocation of net income for basic common stock		$ 6,872	$ 4,227
Weighted average number of shares for basic:
Weighted-average number of common shares - basic		8,714	9,230
Net income per share for basic:
Net income per common share - basic		$ 0.79	$ 0.46
Common Class B
Allocation of net income for basic:
Allocation of net income for basic common stock		$ 2,380	$ 1,452
Weighted average number of shares for basic:
Weighted-average number of common shares - basic	[1]	3,018	3,170
Net income per share for basic:
Net income per common share - basic		$ 0.79	$ 0.46
Unvested Participating Shares
Weighted average number of shares for basic:
Weighted-average number of common shares - basic		73	71

[1] Change in Class B Common Stock resulted from the conversion of 79,000 shares to Class A Common Stock on October 31, 2017 with the ending number of shares reflecting the weighted average for the period.