Income Per Share of Common Stock (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Income per share of common stock:
Net income for basic and diluted income per common share	$ 6,033	$ 5,128	$ 6,638	$ 4,262	$ 11,161	$ 10,900
Weighted-average shares for basic income per common share (in shares)	5,614		5,595		5,612	5,593
Effect of dilutive securities:
Employee and director stock options (in shares)	10		8		7	8
Adjusted weighted-average shares and assumed exercises for diluted income per common share (in shares)	5,624		5,603		5,619	5,601
Basic net income per common share (in dollars per share)	$ 1.07		$ 1.19		$ 1.99	$ 1.95
Diluted net income per common share (in dollars per share)	$ 1.07		$ 1.18		$ 1.99	$ 1.95
Options
Anti-dilutive securities
Anti-dilutive shares of common stock excluded from the calculation of dilutive securities	0		2		14	10