Income Per Share of Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income per share of common stock:
Net income for basic and diluted income per common share	$ 4,262	$ 4,620
Weighted-average shares for basic income per common share	5,590,000	5,576,000
Effect of dilutive securities:
Employee and director stock options (in shares)	8,000	9,000
Adjusted weighted-average shares and assumed exercises for diluted income per common share	5,598,000	5,585,000
Basic net income per common share (in dollars per share)	$ 0.76	$ 0.83
Diluted net income per common share (in dollars per share)	$ 0.76	$ 0.83
Options
Anti-dilutive securities
Anti-dilutive shares of common stock excluded from the calculation of dilutive securities	17,400