Fair Value Measurements (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Gains and losses on equity instruments
Net (losses) gains recognized during the period on equity investments	$ (6,000)	$ 54,000	$ 86,000	$ 4,000
Less: net gains recognized during the period on equity investments sold during the period	5,000	14,000	5,000	37,000
Unrealized (losses) gains recognized during the period on equity investments still held at period end	(11,000)	$ 40,000	81,000	$ (33,000)
Estimate of Fair Value, Fair Value Disclosure
Fair Value Measurements
Investments	1,406,000		1,406,000		$ 1,322,000
Fair Value, Inputs, Level 1
Fair Value Measurements
Investments	$ 1,406,000		$ 1,406,000		$ 1,322,000