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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net income (loss)									$ (1,913)	$ 13,279	$ 13,690
Less: Income (loss) allocated to unvested participating securities									(21)	292	256
Net income (loss) available to common stockholders									$ (1,892)	$ 12,987	$ 13,434
Denominator:
Denominator for basic earnings (loss) per share - weighted average shares	5,880	5,869	5,868	5,866	5,817	5,834	5,844	5,841	5,871	5,834	5,829
Effect of dilutive securities:
Denominator for diluted earnings per share - adjusted weighted-average shares and assumed conversions	5,880	5,869	5,868	5,866	5,817	5,834	5,844	5,841	5,871	5,834	5,829
Basic earnings (loss) per share:
Basic	$ 0.38	$ (0.16)	$ (0.82)	$ 0.28	$ 0.64	$ 0.56	$ 0.80	$ 0.23	$ (0.32)	$ 2.23	$ 2.30
Diluted earnings (loss) per share
Diluted earning (loss) per share	$ 0.38	$ (0.16)	$ (0.82)	$ 0.28	$ 0.64	$ 0.56	$ 0.80	$ 0.23	$ (0.32)	$ 2.23	$ 2.30