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, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Numerator:
Net income (loss)									$ 11,157	$ (1,913)	$ 13,279
Less: Income (loss) allocated to unvested participating securities									190	(21)	292
Net income (loss) available to common stockholders									$ 10,967	$ (1,892)	$ 12,987
Denominator:
Weighted average common shares	5,922	5,917	5,917	5,913	5,880	5,869	5,868	5,866	5,917	5,871	5,834
Effect of dilutive securities:
Denominator for diluted earnings per share - adjusted weighted-average shares and assumed conversions	5,922	5,917	5,917	5,913	5,880	5,869	5,868	5,866	5,917	5,871	5,834
Basic earnings (loss) per share
Basic	$ 0.60	$ 0.58	$ 0.54	$ 0.13	$ 0.38	$ (0.16)	$ (0.82)	$ 0.28	$ 1.85	$ (0.32)	$ 2.23
Diluted earnings (loss) per share
Diluted earning per share	$ 0.60	$ 0.58	$ 0.54	$ 0.13	$ 0.38	$ (0.16)	$ (0.82)	$ 0.28	$ 1.85	$ (0.32)	$ 2.23