Quarterly Results of Operations - Quarterly Results of Operations (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information [Line Items]
Interest income	$ 21,062	$ 21,624	$ 21,554	$ 21,131	$ 20,947	$ 20,549	$ 19,132	$ 19,137	$ 85,371	$ 79,765	$ 81,494
Interest expense	4,840	4,879	4,800	4,617	4,842	4,751	4,620	4,592	19,136	18,805	19,540
Net interest income	16,222	16,745	16,754	16,514	16,105	15,798	14,512	14,545	66,235	60,960	61,954
Provision for loan losses	400	600	450	600	460	750	750	750	2,050	2,710	4,150
Net interest income after provision for loan losses	15,822	16,145	16,304	15,914	15,645	15,048	13,762	13,795	64,185	58,250	57,804
Other operating income	4,428	3,758	3,615	3,470	4,186	4,774	5,221	4,434	15,271	18,615	15,865
Operating expenses	14,506	13,976	14,089	14,159	14,690	13,995	13,662	13,465	56,730	55,812	53,238
Income before income taxes	5,744	5,927	5,830	5,225	5,141	5,827	5,321	4,764	22,726	21,053	20,431
Provision for income taxes	121	221	231	293	116	308	295	288	866	1,007	1,392
Net income	5,623	5,706	5,599	4,932	5,025	5,519	5,026	4,476	21,860	20,046	19,039
Class A Common Stock [Member]
Quarterly Financial Information [Line Items]
Net income									17,157	15,698	14,877
Share data:
Average shares outstanding, basic	3,600,664	3,594,583	3,589,125	3,582,421	3,580,404	3,578,400	3,574,379	3,569,546	3,591,732	3,575,683	3,557,693
Average shares outstanding, diluted	5,567,855	5,563,278	5,558,032	5,558,177	5,557,419	5,558,031	5,557,354	5,557,365	5,562,209	5,557,693	5,549,191
Earnings per share, basic	$ 1.23	$ 1.25	$ 1.22	$ 1.08	$ 1.10	$ 1.21	$ 1.10	$ 0.98	$ 4.78	$ 4.39	$ 4.18
Earnings per share, diluted	$ 1.01	$ 1.03	$ 1.01	$ 0.89	$ 0.90	$ 0.99	$ 0.90	$ 0.81	$ 3.93	$ 3.61	$ 3.43
Class B Common Stock [Member]
Quarterly Financial Information [Line Items]
Net income									$ 4,703	$ 4,348	$ 4,162
Share data:
Average shares outstanding, basic	1,967,180	1,967,180	1,967,580	1,974,180	1,976,180	1,978,180	1,982,180	1,986,880	1,969,030	1,980,855	1,990,474
Average shares outstanding, diluted	1,976,180	1,967,180	1,967,580	1,974,180	1,976,180	1,978,180	1,982,180	1,986,880	1,969,030	1,980,855	1,990,474
Earnings per share, basic	$ 0.61	$ 0.62	$ 0.61	$ 0.54	$ 0.55	$ 0.60	$ 0.55	$ 0.49	$ 2.39	$ 2.19	$ 2.09
Earnings per share, diluted	$ 0.61	$ 0.62	$ 0.61	$ 0.54	$ 0.55	$ 0.60	$ 0.55	$ 0.49	$ 2.39	$ 2.19	$ 2.09