Earnings Per Share ("EPS") - Reconciliation of Basic EPS and Diluted EPS (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
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 5,026	$ 4,771	$ 9,502	$ 8,579
Class A Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	3,935	3,727	7,437	6,702
Weighted average number of shares outstanding, basic	3,574,379	3,556,474	3,571,963	3,553,734
Basic earnings per share	$ 1.10	$ 1.05	$ 2.08	$ 1.89
Dilutive effect of Class A stock options	795	476	1,162	686
Weighted average shares outstanding, diluted	5,557,354	5,548,830	5,557,655	5,547,237
Diluted earnings per share	$ 0.90	$ 0.86	$ 1.71	$ 1.55
Class B Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 1,091	$ 1,044	$ 2,065	$ 1,877
Weighted average number of shares outstanding, basic	1,982,180	1,991,880	1,984,530	1,992,818
Basic earnings per share	$ 0.55	$ 0.52	$ 1.04	$ 0.94
Weighted average shares outstanding, diluted	1,982,180	1,991,880	1,984,530	1,992,818
Diluted earnings per share	$ 0.55	$ 0.52	$ 1.04	$ 0.94