Quarterly Results of Operations - Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information [Line Items]
Interest income	$ 37,453	$ 34,765	$ 33,408	$ 31,430	$ 29,470	$ 28,521	$ 28,806	$ 26,639	$ 137,056	$ 113,436	$ 96,699
Interest expense	13,748	11,561	10,209	8,962	7,768	7,168	6,701	6,183	44,480	27,820	22,617
Net interest income	23,705	23,204	23,199	22,468	21,702	21,353	22,105	20,456	92,576	85,616	74,082
Provision for loan losses	450		450	450	450	450	490	400	1,350	1,790	1,375
Net interest income after provision for loan losses	23,255	23,204	22,749	22,018	21,252	20,903	21,615	20,056	91,226	83,826	72,707
Other operating income	4,164	4,169	3,722	4,193	4,410	3,942	4,291	3,909	16,248	16,552	16,222
Operating expenses	17,185	17,348	17,159	18,001	15,992	16,205	17,197	17,725	69,693	67,119	64,757
Income before income taxes	10,234	10,025	9,312	8,210	9,670	8,640	8,709	6,240	37,781	33,259	24,172
Provision for income taxes	309	444	314	501	9,645	617	552	144	1,568	10,958	(362)
Net income	$ 9,925	$ 9,581	$ 8,998	$ 7,709	$ 25	$ 8,023	$ 8,157	$ 6,096	$ 36,213	$ 22,301	$ 24,534
Class A Common Stock [Member]
Share data:
Average shares outstanding, basic	3,608,329	3,608,329	3,608,029	3,608,029	3,605,829	3,605,829	3,603,729	3,600,729	3,608,179	3,604,029	3,600,729
Average shares outstanding, diluted	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909
Earnings per share, basic	$ 2.16	$ 2.09	$ 1.96	$ 1.68	$ 0.01	$ 1.75	$ 1.78	$ 1.33	$ 7.89	$ 4.86	$ 5.35
Earnings per share, diluted	$ 1.78	$ 1.72	$ 1.62	$ 1.38		$ 1.44	$ 1.47	$ 1.09	$ 6.50	$ 4.01	$ 4.41
Class B Common Stock [Member]
Share data:
Average shares outstanding, basic	1,959,580	1,959,580	1,959,880	1,959,880	1,962,080	1,962,080	1,964,180	1,967,180	1,959,730	1,963,880	1,967,180
Average shares outstanding, diluted	1,959,580	1,959,580	1,959,880	1,959,880	1,962,080	1,962,080	1,964,180	1,967,180	1,959,730	1,963,880	1,967,180
Earnings per share, basic	$ 1.08	$ 1.04	$ 0.98	$ 0.84		$ 0.87	$ 0.89	$ 0.66	$ 3.95	$ 2.43	$ 2.68
Earnings per share, diluted	$ 1.08	$ 1.04	$ 0.98	$ 0.84		$ 0.87	$ 0.89	$ 0.66	$ 3.95	$ 2.43	$ 2.68