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The following table presents additional information about marketable equity securities: (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Net Investment Income [Line Items]          
Purchases of marketable equity securities     $ 4,994,000 $ 517,000  
Sales of marketable equity securities     (430,000) (144,000)  
Realized gains on marketable equity securities $ 30,000 428,000  
Realized losses on marketable equity securities     15,154,000 23,000  
Unrealized gains on marketable equity securities (750,000) $ (29,000) (705,000) $ (58,000)  
Common Stock [Member]          
Net Investment Income [Line Items]          
Balance at January 1, 2021     2,563,000    
Purchases of marketable equity securities in operations     338,620,000    
Purchases of marketable equity securities     2,144,000   $ 1,506,000
Conversion of debt securities to marketable securities     2,656,000    
Sales of marketable equity securities in operations     (303,923,000)    
Sales of marketable equity securities     (430,000)    
Realized gains on marketable equity securities     19,040,000    
Realized losses on marketable equity securities     (4,386,000)    
Unrealized gains on marketable equity securities     (6,353,000)    
Balance at September 30, 2021 $ 49,931,000   $ 49,931,000   $ 2,563,000