Earnings Per Share ("EPS") - Reconciliation of Basic EPS and Diluted EPS (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 9,466	$ 8,998	$ 18,884	$ 16,707
Class A Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 7,460	$ 7,076	$ 14,867	$ 13,138
Weighted average shares outstanding, basic	3,620,449	3,608,029	3,615,389	3,608,029
Basic earnings per share	$ 2.06	$ 1.96	$ 4.11	$ 3.64
Net income	$ 7,460	$ 7,076	$ 14,867	$ 13,138
Weighted average shares outstanding, diluted	5,567,909	5,567,909	5,567,909	5,567,909
Diluted earnings per share	$ 1.70	$ 1.62	$ 3.39	$ 3.00
Class B Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 2,006	$ 1,922	$ 4,017	$ 3,569
Weighted average shares outstanding, basic	1,947,460	1,959,880	1,952,520	1,959,880
Basic earnings per share	$ 1.03	$ 0.98	$ 2.06	$ 1.82
Net income	$ 2,006	$ 1,922	$ 4,017	$ 3,569
Weighted average shares outstanding, diluted	1,947,460	1,959,880	1,952,520	1,959,880
Diluted earnings per share	$ 1.03	$ 0.98	$ 2.06	$ 1.82