Summary of Significant Accounting Policies - Basic and diluted earnings per share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Income from continuing operations	$ 3,841	$ 3,334	$ 4,734	$ 1,370	$ 4,295	$ 3,695	$ 4,171	$ 1,529	$ 13,279	$ 13,690	$ 22,246
Less: Income allocated to unvested participating securities									292	256	370
Income from continuing operations available to common stockholders									12,987	13,434	21,876
Income from discontinued operations									0	0	32,471
Less: Income allocated to unvested participating securities									0	0	541
Income from discontinued operations available to common stockholders									0	0	31,930
Net income available to common stockholders									$ 12,987	$ 13,434	$ 53,806
Denominator:
Denominator for basic earnings per share-weighted average shares	5,817	5,834	5,844	5,841	5,820	5,822	5,834	5,842	5,834	5,829	5,803
Effect of dilutive securities:
Stock options									0	0	4
Denominator for diluted earnings per share - adjusted weighted-average shares and assumed conversions	5,817	5,834	5,844	5,841	5,820	5,822	5,834	5,842	5,834	5,829	5,807
Basic earnings per share:
From continuing operations									$ 2.23	$ 2.30	$ 3.77
From discontinued operations									0	0.00	5.50
Earnings Per Share, Basic, Total	$ 0.64	$ 0.56	$ 0.80	$ 0.23	$ 0.72	$ 0.62	$ 0.70	$ 0.26	2.23	2.30	9.27
Diluted earnings per share
From continuing operations									2.23	2.30	3.77
From discontinued operations									0	0.00	5.50
Earnings Per Share, Diluted, Total	$ 0.64	$ 0.56	$ 0.80	$ 0.23	$ 0.72	$ 0.62	$ 0.70	$ 0.26	$ 2.23	$ 2.30	$ 9.27