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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 8,157	$ 6,241	$ 14,253	$ 11,549
Class A Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 6,410	$ 4,902	$ 11,198	$ 9,071
Weighted average shares outstanding, basic	3,603,729	3,600,729	3,602,229	3,600,729
Basic earnings per share	$ 1.78	$ 1.36	$ 3.11	$ 2.52
Net income	$ 6,410	$ 4,902	$ 11,198	$ 9,071
Weighted average shares outstanding, diluted	5,567,909	5,567,909	5,567,909	5,567,909
Diluted earnings per share	$ 1.47	$ 1.12	$ 2.56	$ 2.07
Class B Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 1,747	$ 1,339	$ 3,055	$ 2,478
Weighted average shares outstanding, basic	1,964,180	1,967,180	1,965,680	1,967,180
Basic earnings per share	$ 0.89	$ 0.68	$ 1.55	$ 1.26
Net income	$ 1,747	$ 1,339	$ 3,055	$ 2,478
Weighted average shares outstanding, diluted	1,964,180	1,967,180	1,965,680	1,967,180
Diluted earnings per share	$ 0.89	$ 0.68	$ 1.55	$ 1.26