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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 |  |  |

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