Summary of Significant Accounting Policies - Basic and diluted earnings per share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net income (loss)	$ (4,911)	$ 4,734	$ (3,231)	$ 6,104
Less: Income (loss) allocated to unvested participating securities	(104)	88	(68)	114
Net income available to common stockholders	$ (4,807)	$ 4,646	$ (3,163)	$ 5,990
Denominator:
Denominator for basic earnings (loss) per share - weighted average shares	5,868	5,844	5,867	5,843
Effect of dilutive securities:
Common stock equivalents	0	0	0	0
Denominator for diluted earnings per share - adjusted weighted-average shares and assumed conversions	5,868	5,844	5,867	5,843
Basic earnings (loss) per share:
Basic	$ (0.82)	$ 0.80	$ (0.54)	$ 1.03
Diluted earnings (loss) per share
Diluted earning (loss) per share	$ (0.82)	$ 0.80	$ (0.54)	$ 1.03

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