Summary of Significant Accounting Policies - Basic and diluted earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net income	$ 1,680	$ 1,370
Less: Income allocated to unvested participating securities	37	26
Net income available to common stockholders	$ 1,643	$ 1,344
Denominator:
Denominator for basic earnings per share - weighted average shares	5,866	5,841
Effect of dilutive securities:
Common stock equivalents	0	0
Denominator for diluted earnings per share - adjusted weighted-average shares and assumed conversions	5,866	5,841
Basic earnings per share:
Basic	$ 0.28	$ 0.23
Diluted earnings per share
Diluted	$ 0.28	$ 0.23

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