Earnings Per Share ("EPS") - Reconciliation of Basic EPS and Diluted EPS (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 5,519	$ 5,682	$ 15,021	$ 14,261
Class A Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	4,324	4,441	11,760	11,142
Weighted average number of shares outstanding, basic	3,578,400	3,559,125	3,574,109	3,555,536
Basic earnings per share	$ 1.21	$ 1.25	$ 3.29	$ 3.13
Dilutive effect of Class A stock options	1,451	1,305	1,261	926
Weighted average shares outstanding, diluted	5,558,031	5,549,810	5,557,783	5,548,133
Diluted earnings per share	$ 0.99	$ 1.02	$ 2.70	$ 2.57
Class B Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 1,195	$ 1,241	$ 3,261	$ 3,119
Weighted average number of shares outstanding, basic	1,978,180	1,989,380	1,982,413	1,991,671
Basic earnings per share	$ 0.60	$ 0.62	$ 1.64	$ 1.57
Weighted average shares outstanding, diluted	1,978,180	1,989,380	1,982,413	1,991,671
Diluted earnings per share	$ 0.60	$ 0.62	$ 1.64	$ 1.57