Earnings Per Share ("EPS") - Reconciliation of Basic EPS and Diluted EPS (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
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
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]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income									$ 31,351	$ 28,479	$ 17,526
Weighted 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
Basic earnings per share	$ 2.33	$ 2.19	$ 2.06	$ 2.05	$ 2.16	$ 2.09	$ 1.96	$ 1.68	$ 8.63	$ 7.89	$ 4.86
Net income									$ 31,351	$ 28,479	$ 17,526
Weighted 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
Diluted earnings per share	$ 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]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income									$ 8,348	$ 7,734	$ 4,775
Weighted 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
Basic earnings per share	$ 1.16	$ 1.09	$ 1.03	$ 1.03	$ 1.08	$ 1.04	$ 0.98	$ 0.84	$ 4.31	$ 3.95	$ 2.43
Net income									$ 8,348	$ 7,734	$ 4,775
Weighted 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
Diluted earnings per share	$ 1.16	$ 1.09	$ 1.03	$ 1.03	$ 1.08	$ 1.04	$ 0.98	$ 0.84	$ 4.31	$ 3.95	$ 2.43