Earnings Per Share ("EPS") - Reconciliation of Basic EPS and Diluted EPS (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 9,418	$ 7,709
Class A Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 7,409	$ 6,062
Weighted average shares outstanding, basic	3,610,329	3,608,029
Basic earnings per share	$ 2.05	$ 1.68
Net income	$ 7,409	$ 6,062
Weighted average shares outstanding, diluted	5,567,909	5,567,909
Diluted earnings per share	$ 1.69	$ 1.38
Class B Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 2,009	$ 1,647
Weighted average shares outstanding, basic	1,957,580	1,959,880
Basic earnings per share	$ 1.03	$ 0.84
Net income	$ 2,009	$ 1,647
Weighted average shares outstanding, diluted	1,957,580	1,959,880
Diluted earnings per share	$ 1.03	$ 0.84