Summary of Significant Accounting Policies - Basic and diluted earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Numerator:
Net income									$ 9,500	$ 9,202	$ 11,157
Less: Income allocated to unvested participating securities									149	140	190
Net income available to common shareholders									$ 9,351	$ 9,062	$ 10,967
Denominator:
Denominator for basic earnings per share - weighted average shares	6,030,000	6,032,000	6,032,000	6,028,000	6,013,000	5,961,000	5,952,000	5,948,000	6,045,000	5,973,000	5,917,000
Effect of dilutive securities:
Denominator for diluted earnings per share - adjusted weighted-average shares and assumed conversions	6,030,000	6,032,000	6,032,000	6,028,000	6,013,000	5,961,000	5,952,000	5,948,000	6,045,000	5,973,000	5,917,000
Earnings per share:
Basic (in dollars per share)	$ 0.40	$ 0.45	$ 0.55	$ 0.15	$ 0.70	$ (0.01)	$ 0.63	$ 0.20	$ 1.55	$ 1.52	$ 1.85
Diluted earnings per share:
Diluted (in dollars per share)	$ 0.40	$ 0.45	$ 0.55	$ 0.15	$ 0.70	$ (0.01)	$ 0.63	$ 0.20	$ 1.55	$ 1.52	$ 1.85
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount									0	0	0