Income Per Share of Common Stock (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Income per share of common stock:
Net income for basic and diluted income per common share	$ 6,101	$ 5,544	$ 6,033	$ 5,128	$ 11,645	$ 11,161
Weighted-average shares for basic income per common share (in shares)	5,629		5,614		5,627	5,612
Effect of dilutive securities:
Employee and director stock options (in shares)	8		10		9	7
Adjusted weighted-average shares and assumed exercises for diluted income per common share (in shares)	5,637		5,624		5,636	5,619
Basic net income per common share (in dollars per share)	$ 1.08		$ 1.07		$ 2.07	$ 1.99
Diluted net income per common share (in dollars per share)	$ 1.08		$ 1.07		$ 2.07	$ 1.99
Options
Anti-dilutive securities
Anti-dilutive shares of common stock excluded from the calculation of dilutive securities	15		0		11	14