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Marketable Securities (Details) - USD ($)
 3 Months Ended 12 Months Ended
Jun. 28, 2013
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Marketable Securities        
Gross proceeds from sale of marketable securities   $ 31,631,000 $ 34,591,000  
Gain (loss) on marketable securities   1,950,000 (2,204,000)  
Unrealized (gains) loss on marketable securities   $ 399,000 $ 0  
Common Stock        
Marketable Securities        
Amount recognized in connection with the earn-out including other income (loss) (in shares)       992,247
Number of shares sold in transaction (in shares)   223,608 343,562  
Gross proceeds from sale of marketable securities   $ 31,600,000 $ 34,600,000  
Gain (loss) on marketable securities   2,000,000.0 (2,200,000)  
Unrealized (gains) loss on marketable securities   400,000 $ 0  
Marketable securities   $ 4,000,000.0   $ 33,300,000
Maximum | Common Stock        
Marketable Securities        
Remaining earn-out receivable under common stock transaction (in shares) 6,945,729      
Minimum | Common Stock        
Marketable Securities        
Gross revenue on earn-out receivable per year under common stock transaction $ 25,000,000.0      
BGC Partners Inc | Common Stock        
Marketable Securities        
Period for earn-out receivable under common stock transaction (years)       7 years
BGC Partners Inc | Nasdaq Omx        
Marketable Securities        
Period for earn-out receivable under common stock transaction (years) 15 years      
BGC Partners Inc | Nasdaq Omx | Maximum        
Marketable Securities        
Earn-out shares receivable under common stock transaction (in shares) 14,883,705      
BGC Partners Inc | Nasdaq Omx | Minimum        
Marketable Securities        
Gross revenue on earn-out receivable per year under common stock transaction $ 25,000,000.0