Income Per Share of Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income per share of common stock:
Net income for basic and diluted income per common share	$ 5,667	$ 6,081	$ 15,565	$ 13,834
Weighted-average shares for basic income per common share	5,577,000	5,565,000	5,577,000	5,569,000
Effect of dilutive securities:
Employee and director stock options (in shares)	10,000	4,000	9,000	3,000
Adjusted weighted-average shares and assumed exercises for diluted income per common share	5,587,000	5,569,000	5,586,000	5,572,000
Basic net income per common share (in dollars per share)	$ 1.02	$ 1.09	$ 2.79	$ 2.48
Diluted net income per common share (in dollars per share)	$ 1.01	$ 1.09	$ 2.79	$ 2.48
Options
Anti-dilutive securities
Anti-dilutive shares of common stock excluded from the calculation of dilutive securities		7,500		7,500