Quarterly Results of Operations - Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information [Line Items]
Interest income	$ 40,518	$ 39,852	$ 39,692	$ 39,077	$ 37,453	$ 34,765	$ 33,408	$ 31,430	$ 159,139	$ 137,056	$ 113,436
Interest expense	15,187	16,082	16,442	15,639	13,748	11,561	10,209	8,962	63,350	44,480	27,820
Net interest income	25,331	23,770	23,250	23,438	23,705	23,204	23,199	22,468	95,789	92,576	85,616
Provision for loan losses	550	75	250	375	450		450	450	1,250	1,350	1,790
Net interest income after provision for loan losses	24,781	23,695	23,000	23,063	23,255	23,204	22,749	22,018	94,539	91,226	83,826
Other operating income	4,689	4,286	4,997	4,427	4,164	4,169	3,722	4,193	18,399	16,248	16,552
Operating expenses	18,212	17,462	18,264	18,190	17,185	17,348	17,159	18,001	72,129	69,693	67,119
Income before income taxes	11,258	10,519	9,733	9,300	10,234	10,025	9,312	8,210	40,809	37,781	33,259
Provision for income taxes	526	435	267	(118)	309	444	314	501	1,110	1,568	10,958
Net income	$ 10,732	$ 10,084	$ 9,466	$ 9,418	$ 9,925	$ 9,581	$ 8,998	$ 7,709	$ 39,699	$ 36,213	$ 22,301
Class A Common Stock [Member]
Share data:
Average shares outstanding, basic	3,650,949	3,650,449	3,620,449	3,610,329	3,608,329	3,608,329	3,608,029	3,608,029	3,633,044	3,608,179	3,604,029
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	$ 2.33	$ 2.19	$ 2.06	$ 2.05	$ 2.16	$ 2.09	$ 1.96	$ 1.68	$ 8.63	$ 7.89	$ 4.86
Earnings per share, diluted	$ 1.93	$ 1.81	$ 1.70	$ 1.69	$ 1.78	$ 1.72	$ 1.62	$ 1.38	$ 7.13	$ 6.50	$ 4.01
Class B Common Stock [Member]
Share data:
Average shares outstanding, basic	1,916,960	1,917,460	1,947,460	1,957,580	1,959,580	1,959,580	1,959,880	1,959,880	1,934,865	1,959,730	1,963,880
Average shares outstanding, diluted	1,916,960	1,917,460	1,947,460	1,957,580	1,959,580	1,959,580	1,959,880	1,959,880	1,934,865	1,959,730	1,963,880
Earnings per share, basic	$ 1.16	$ 1.09	$ 1.03	$ 1.03	$ 1.08	$ 1.04	$ 0.98	$ 0.84	$ 4.31	$ 3.95	$ 2.43
Earnings per share, diluted	$ 1.16	$ 1.09	$ 1.03	$ 1.03	$ 1.08	$ 1.04	$ 0.98	$ 0.84	$ 4.31	$ 3.95	$ 2.43