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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 6,500	$ 6,486	$ 6,241	$ 5,307	$ 5,971	$ 6,166	$ 5,925	$ 4,959	$ 24,534	$ 23,021	$ 21,860
Class A Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income									$ 19,270	$ 18,081	$ 17,157
Weighted average shares outstanding, basic	3,600,729	3,600,729	3,600,729	3,600,729	3,600,729	3,600,729	3,600,729	3,600,729	3,600,729	3,600,729	3,591,732
Basic earnings per share	$ 1.42	$ 1.41	$ 1.36	$ 1.16	$ 1.30	$ 1.35	$ 1.29	$ 1.08	$ 5.35	$ 5.02	$ 4.78
Dilutive effect of Class A stock options									0	0	1,447
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	5,562,209
Diluted earnings per share	$ 1.17	$ 1.16	$ 1.12	$ 0.95	$ 1.07	$ 1.11	$ 1.06	$ 0.89	$ 4.41	$ 4.13	$ 3.93
Class B Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income									$ 5,264	$ 4,940	$ 4,703
Weighted average shares outstanding, basic	1,967,180	1,967,180	1,967,180	1,967,180	1,967,180	1,967,180	1,967,180	1,967,180	1,967,180	1,967,180	1,969,030
Basic earnings per share	$ 0.71	$ 0.71	$ 0.68	$ 0.58	$ 0.65	$ 0.67	$ 0.65	$ 0.54	$ 2.68	$ 2.51	$ 2.39
Weighted average shares outstanding, diluted	1,967,180	1,967,180	1,967,180	1,967,180	1,976,180	1,967,180	1,967,180	1,967,180	1,967,180	1,967,180	1,969,030
Diluted earnings per share	$ 0.71	$ 0.71	$ 0.68	$ 0.58	$ 0.65	$ 0.67	$ 0.65	$ 0.54	$ 2.68	$ 2.51	$ 2.39