Earnings Per Share - Summary of Reconciliation of Numerators and Denominators Used in Basic and Diluted Per Share Computations (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Common Class A [Member]
Numerator:
Allocation of net income	$ 2,102	$ 1,621	$ 7,304	$ 8,013
Denominator:
Weighted-average shares outstanding	21,202,571	20,041,887	20,637,801	20,019,086
Basic net income per share	$ 0.10	$ 0.08	$ 0.35	$ 0.40
Numerator:
Allocation of net income	2,102	1,621	7,304	8,013
Denominator:
Number of shares used in basic computation	21,202,571	20,041,887	20,637,801	20,019,086
Weighted-average shares effect of dilutive securities
Add: Director and employee stock options	401,309	297,501	346,296	331,169
Number of shares used in per share computations	21,603,880	20,339,388	20,984,097	20,350,255
Diluted net income per share	$ 0.10	$ 0.08	$ 0.35	$ 0.39
Common Class B [Member]
Numerator:
Allocation of net income	527	402	1,801	2,020
Denominator:
Weighted-average shares outstanding	5,576,775	5,576,775	5,576,775	5,576,775
Basic net income per share	$ 0.09	$ 0.07	$ 0.32	$ 0.36
Numerator:
Allocation of net income	$ 527	$ 402	$ 1,801	$ 2,020
Denominator:
Number of shares used in basic computation	5,576,775	5,576,775	5,576,775	5,576,775
Weighted-average shares effect of dilutive securities
Add: Director and employee stock options
Number of shares used in per share computations	5,576,775	5,576,775	5,576,775	5,576,775
Diluted net income per share	$ 0.09	$ 0.07	$ 0.32	$ 0.36