Summary of Significant Accounting Policies - Basic and diluted earnings per share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator:
Net income (loss)	$ 3,454	$ 3,253	$ 758	$ (950)	$ (4,911)	$ 1,680	$ 7,465	$ (4,181)
Less: Income (loss) allocated to unvested participating securities	37			(20)			80	(86)
Net income (loss) available to common stockholders	$ 3,417			$ (930)			$ 7,385	$ (4,095)
Denominator:
Weighted average common shares	5,917			5,869			5,916	5,867
Effect of dilutive securities:
Common stock equivalents	0			0			0	0
Denominator for diluted earnings per share - adjusted weighted-average shares and assumed conversions	5,917			5,869			5,916	5,867
Basic earnings (loss) per share
Basic	$ 0.58			$ (0.16)			$ 1.25	$ (0.70)
Diluted earnings (loss) per share
Diluted earning per share	$ 0.58			$ (0.16)			$ 1.25	$ (0.70)