Income Per Share of Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Apr. 30, 2021	Apr. 29, 2021
Anti-dilutive securities
Common stock, authorized shares	30,000,000				30,000,000		30,000,000	15,000,000
Common stock, shares issued	5,658,481				5,658,481
Income per share of common stock:
Net income for basic and diluted income per common share	$ 11,093	$ 7,031	$ 6,101	$ 5,544	$ 18,124	$ 11,645
Weighted-average shares for basic income per common share (in shares)	5,657,000		5,629,000		5,654,000	5,627,000
Effect of dilutive securities:
Employee and director stock options (in shares)	12,000		8,000		13,000	9,000
Adjusted weighted-average shares and assumed exercises for diluted income per common share (in shares)	5,669,000		5,637,000		5,667,000	5,636,000
Basic net income per common share (in dollars per share)	$ 1.96		$ 1.08		$ 3.21	$ 2.07
Diluted net income per common share (in dollars per share)	$ 1.96		$ 1.08		$ 3.20	$ 2.07
Options
Anti-dilutive securities
Anti-dilutive shares of common stock excluded from the calculation of dilutive securities			15,000			11,000