Marketable securities as of June 30, 2021 and December 31, 2020 were composed of: (Details) - USD ($)	Jun. 30, 2021	Dec. 31, 2020
Cost Basis [Member]
Other Investment Not Readily Marketable [Line Items]
Fixed income mutual funds	$ 12,925,000	$ 11,255,000
Corporate bonds and notes	1,764,000	4,500,000
Total marketable securities	14,689,000	19,289,000
Certificates of deposit		3,534,000
Gross Unrealized Gains [Member]
Other Investment Not Readily Marketable [Line Items]
Fixed income mutual funds	38,000	80,000
Corporate bonds and notes		18,000
Total marketable securities	38,000	105,000
Certificates of deposit		7,000
Fair Value [Member]
Other Investment Not Readily Marketable [Line Items]
Fixed income mutual funds	12,963,000	11,335,000
Corporate bonds and notes	1,762,000	4,490,000
Total marketable securities	14,725,000	19,366,000
Certificates of deposit		3,541,000
Gross Unrealized Losses [Member]
Other Investment Not Readily Marketable [Line Items]
Corporate bonds and notes	(2,000)	(28,000)
Total marketable securities	$ (2,000)	$ (28,000)