Shareholders' Equity - Summary of common shareholders' equity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ 33,737	$ 33,532
Retirement benefits adjustment	15
Purchase of treasury shares	(81)	(100)
Stock based compensation	31	85
Net income	541	1,898
Balance	34,243	35,415
Retained earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	21,803	20,484
Retirement benefits adjustment	0
Purchase of treasury shares	0	0
Stock based compensation	0	0
Net income	541	1,898
Balance	22,344	22,382
Accumulated Other Comprehensive Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	(1,356)	98
Retirement benefits adjustment	15
Purchase of treasury shares	0	0
Stock based compensation	0	0
Net income	0	0
Balance	(1,341)	98
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	523	523
Retirement benefits adjustment	0
Purchase of treasury shares	0	0
Stock based compensation	0	0
Net income	0	0
Balance	523	523
Capital in excess of par value
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	14,481	14,358
Retirement benefits adjustment	0
Purchase of treasury shares	0	0
Stock based compensation	11	33
Net income	0	0
Balance	14,492	14,391
ESOT
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	(359)	(460)
Retirement benefits adjustment	0
Purchase of treasury shares	0	0
Stock based compensation	0	0
Net income	0	0
Balance	(359)	(460)
Treasury stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	(1,355)	(1,471)
Retirement benefits adjustment	0
Purchase of treasury shares	(81)	(100)
Stock based compensation	20	52
Net income	0	0
Balance	$ (1,416)	$ (1,519)