Income Per Share of Common Stock (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Income per share of common stock:
Net income for basic and diluted income per common share	$ 9,324	$ 6,101	$ 5,544	$ 9,902	$ 6,033	$ 5,128	$ 20,969	$ 21,063
Weighted-average shares for basic income per common share (in shares)	5,630			5,613			5,628	5,612
Effect of dilutive securities:
Employee and director stock options (in shares)	11			9			10	8
Adjusted weighted-average shares and assumed exercises for diluted income per common share (in shares)	5,641			5,622			5,637	5,620
Basic net income per common share (in dollars per share)	$ 1.66			$ 1.76			$ 3.73	$ 3.75
Diluted net income per common share (in dollars per share)	$ 1.65			$ 1.76			$ 3.72	$ 3.75
Options
Anti-dilutive securities
Anti-dilutive shares of common stock excluded from the calculation of dilutive securities	0			0			7	9