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		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Class A common stock [Member]
Numerator:
Allocation of net income	$ 7,072	$ 6,133	$ 8,131	$ 13,414
Denominator:
Weighted-average shares outstanding	21,175,388	20,383,165	21,004,426	20,246,449
Basic net income per share	$ 0.33	$ 0.30	$ 0.39	$ 0.66
Numerator:
Allocation of net income	7,072	6,133	8,131	13,414
Denominator:
Number of shares used in basic computation	21,175,388	20,383,165	21,004,426	20,246,449
Weighted-average shares effect of dilutive securities
Add: Director and employee stock options	526,260	224,446	431,762	305,377
Number of shares used in per share computations	21,701,648	20,607,611	21,436,188	20,551,826
Diluted net income per share	$ 0.33	$ 0.30	$ 0.38	$ 0.65
Class B common stock [Member]
Numerator:
Allocation of net income	1,677	1,521	1,922	3,344
Denominator:
Weighted-average shares outstanding	5,576,775	5,576,775	5,576,775	5,576,775
Basic net income per share	$ 0.30	$ 0.27	$ 0.35	$ 0.60
Numerator:
Allocation of net income	$ 1,677	$ 1,521	$ 1,922	$ 3,344
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	0		0
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	$ 0.30	$ 0.27	$ 0.35	$ 0.60