Shareholders' Equity - Summary of common shareholders' equity (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ 33,126,000	$ 31,916,000	$ 31,541,000	$ 31,513,000	$ 30,280,000	$ 28,722,000	$ 28,379,000	$ 28,165,000	$ 31,513,000	$ 28,165,000
Net income	165,000	1,132,000	714,000	98,000	1,003,000	1,457,000	707,000	331,000	2,109,000	3,498,000
Compensation expense					101,000				101,000	101,000
Retirement benefits adjustment	164,000				67,000				63,000	67,000
Purchase of treasury shares		(8,000)	(21,000)	(128,000)	(25,000)		(33,000)	(117,000)
Dividends declared ($0.16 per share)			(413,000)			3,000	(416,000)
Stock based compensation	(77,000)	86,000	95,000	(58,000)	87,000	98,000	85,000
Balance	$ 33,532,000	33,126,000	31,916,000	31,541,000	$ 31,513,000	30,280,000	28,722,000	28,379,000	$ 33,532,000	$ 31,513,000
Balance (shares)	2,614,506				2,614,506				2,614,506	2,614,506
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	$ 523,000	523,000	523,000	523,000	$ 523,000	523,000	523,000	523,000	$ 523,000	$ 523,000
Stock based compensation			0
Balance	523,000	523,000	523,000	523,000	523,000	523,000	523,000	523,000	523,000	523,000
Capital in excess of par value
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	14,326,000	14,298,000	14,264,000	14,250,000	14,226,000	14,192,000	14,171,000	14,171,000	14,250,000	14,171,000
Stock based compensation	(32,000)	28,000	34,000	(14,000)	24,000	34,000	21,000
Balance	14,358,000	14,326,000	14,298,000	14,264,000	14,250,000	14,226,000	14,192,000	14,171,000	14,358,000	14,250,000
Retained earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	20,319,000	19,187,000	18,886,000	18,788,000	17,785,000	16,325,000	16,040,000	15,709,000	18,788,000	15,709,000
Net income	165,000	1,132,000	714,000	98,000	1,003,000	1,457,000	707,000	331,000
Dividends declared ($0.16 per share)			(413,000)			3,000	(416,000)
Stock based compensation			0				(6,000)
Balance	20,484,000	20,319,000	19,187,000	18,886,000	18,788,000	17,785,000	16,325,000	16,040,000	20,484,000	18,788,000
ESOT
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	(561,000)	(561,000)	(561,000)	(561,000)	(662,000)	(662,000)	(662,000)	(662,000)	(561,000)	(662,000)
Compensation expense					101,000
Retirement benefits adjustment	101,000
Stock based compensation			0
Balance	(460,000)	(561,000)	(561,000)	(561,000)	(561,000)	(662,000)	(662,000)	(662,000)	(460,000)	(561,000)
Treasury stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	(1,516,000)	(1,566,000)	(1,606,000)	(1,522,000)	(1,566,000)	(1,630,000)	(1,661,000)	(1,544,000)	(1,522,000)	(1,544,000)
Purchase of treasury shares		(8,000)	(21,000)	(128,000)	(25,000)		(33,000)	(117,000)
Stock based compensation	(45,000)	58,000	61,000	(44,000)	69,000	64,000	64,000
Balance	(1,471,000)	(1,516,000)	(1,566,000)	(1,606,000)	(1,522,000)	(1,566,000)	(1,630,000)	(1,661,000)	(1,471,000)	(1,522,000)
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance	35,000	35,000	35,000	35,000	(26,000)	(26,000)	(32,000)	(32,000)	35,000	(32,000)
Retirement benefits adjustment	63,000				67,000
Stock based compensation			0		(6,000)		6,000
Balance	$ 98,000	$ 35,000	$ 35,000	$ 35,000	$ 35,000	$ (26,000)	$ (26,000)	$ (32,000)	$ 98,000	$ 35,000