Income Per Share of Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 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	5,658,467						5,658,467
Income per share of common stock:
Net income for basic and diluted income per common share	$ 11,308	$ 11,093	$ 7,031	$ 9,324	$ 6,101	$ 5,544	$ 29,432	$ 20,969
Weighted-average shares for basic income per common share (in shares)	5,658,000			5,630,000			5,656,000	5,628,000
Effect of dilutive securities:
Employee and director stock options (in shares)	13,000			11,000			12,000	10,000
Adjusted weighted-average shares and assumed exercises for diluted income per common share (in shares)	5,671,000			5,641,000			5,668,000	5,638,000
Basic net income per common share (in dollars per share)	$ 2.00			$ 1.66			$ 5.20	$ 3.73
Diluted net income per common share (in dollars per share)	$ 1.99			$ 1.65			$ 5.19	$ 3.72
Options
Anti-dilutive securities
Anti-dilutive shares of common stock excluded from the calculation of dilutive securities				0				7,000