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Fair Value Measurements (Details) - USD ($)	3 Months Ended		6 Months Ended
Fair Value Measurements (Details) - USD ($)	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Gains and losses on equity instruments
Net gains (losses) recognized during the period on equity investments	$ 26,000	$ 10,000	$ 113,000	$ (6,000)
Less: net gains recognized during the period on equity investments sold during the period		(5,000)		(13,000)
Unrealized gains (losses) recognized during the period on equity investments still held at period end	26,000	$ 5,000	113,000	$ (19,000)
Total
Fair Value Measurements
Equity investments	1,118,000		1,118,000		$ 995,000
Level 1
Fair Value Measurements
Equity investments	$ 1,118,000		$ 1,118,000		$ 995,000