Shareholders' Equity - Summary of common shareholders' equity (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ 36,467	$ 35,415	$ 33,532	$ 31,916	$ 31,541	$ 31,513	$ 33,532	$ 31,513
Dividends declared ($0.16 per share)					(413)
Purchase of treasury shares			(100)	(8)	(21)	(128)
Stock based compensation	83	87	85	86	95	58
Net income (loss)	(1,782)	965	1,898	1,132	714	98	1,081	1,944
Balance	34,768	36,467	35,415	33,126	31,916	31,541	34,768	33,126
Retained earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	23,347	22,382	20,484	19,187	18,886	18,788	20,484	18,788
Dividends declared ($0.16 per share)					(413)
Purchase of treasury shares			0	0	0	0
Stock based compensation	0	0	0	0	0	0
Net income (loss)	(1,782)	965	1,898	1,132	714	98
Balance	21,565	23,347	22,382	20,319	19,187	18,886	21,565	20,319
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	98	98	98	35	35	35	98	35
Dividends declared ($0.16 per share)					0
Purchase of treasury shares			0	0	0	0
Stock based compensation	0	0	0	0	0	0
Net income (loss)	0	0	0	0	0	0
Balance	98	98	98	35	35	35	98	35
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	523	523	523	523	523	523	523	523
Dividends declared ($0.16 per share)					0
Purchase of treasury shares			0	0	0	0
Stock based compensation	0	0	0	0	0	0
Net income (loss)	0	0	0	0	0	0
Balance	523	523	523	523	523	523	523	523
Capital in excess of par value
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	14,430	14,391	14,358	14,298	14,264	14,250	14,358	14,250
Dividends declared ($0.16 per share)					0
Purchase of treasury shares			0	0	0	0
Stock based compensation	29	39	33	28	34	14
Net income (loss)	0	0	0	0	0	0
Balance	14,459	14,430	14,391	14,326	14,298	14,264	14,459	14,326
ESOT
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	(460)	(460)	(460)	(561)	(561)	(561)	(460)	(561)
Dividends declared ($0.16 per share)					0
Purchase of treasury shares			0	0	0	0
Stock based compensation	0	0	0	0	0	0
Net income (loss)	0	0	0	0	0	0
Balance	(460)	(460)	(460)	(561)	(561)	(561)	(460)	(561)
Treasury stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	(1,471)	(1,519)	(1,471)	(1,566)	(1,606)	(1,522)	(1,471)	(1,522)
Dividends declared ($0.16 per share)					0
Purchase of treasury shares			(100)	(8)	(21)	(128)
Stock based compensation	54	48	52	58	61	44
Net income (loss)	0	0	0	0	0	0
Balance	$ (1,417)	$ (1,471)	$ (1,519)	$ (1,516)	$ (1,566)	$ (1,606)	$ (1,417)	$ (1,516)