Stockholders' Equity - Stock incentive plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2014	Jul. 28, 2004
Stockholders Equity
Maximum number of shares authorized for grant				2,000,000
Number of shares available for granting options or stock awards	1,176,000
Share Repurchase Authorization 2004 [Member]
Stockholders Equity
Number of shares of common stock authorized to be repurchased					2,000,000
Number of shares purchased and retired		315,840	308,928
Average purchase price of shares purchased and retired (in dollars per share)		$ 1.99	$ 2.08
Remaining number of shares authorized to be repurchased	384,809
Restricted Stock [Member]
Stockholders Equity
Number of shares purchased and retired	50,572	48,457	47,236
Average purchase price of shares purchased and retired (in dollars per share)	$ 1.86	$ 1.99	$ 2.00
Vesting rights percentage each year beginning on the second anniversary date of the grant				20.00%
Service period over which the aggregate market value of stock is being amortized	6 years			6 years
Number of Shares
Nonvested at the beginning of the period (in shares)	574,800
Granted (in shares)	158,000
Vested (in shares)	(143,800)
Forfeited (in shares)	(53,000)
Nonvested at the end of the period (in shares)	536,000	574,800
Weighted Average Grant Date Fair Value
Nonvested at the beginning of the period (in dollars per share)	$ 2.17
Granted (in dollars per share)	1.86	$ 1.99	$ 2.00
Vested (in dollars per share)	2.29
Forfeited (in dollars per share)	2.07
Nonvested at the end of the period (in dollars per share)	$ 2.05	$ 2.17
Additional disclosure
Total fair value of shares vested during the period (in dollars)	$ 329,000	$ 328,000	$ 288,000
Stock-based compensation expense	311,000	$ 294,000	$ 302,000
Total unrecognized compensation cost (in dollars)	$ 520,000
Weighted-average period for recognition of total unrecognized compensation cost	3 years 7 months 6 days