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Schedule I - Parent Company Only - Organization and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Description Of Business [Line Items] |  |  |
Dividends declared per share to BGC Class A and Class B common stockholders | $ 0.17 | $ 0.56 | $ 0.72
Dividends paid per share to BGC Class A and Class B common stockholders | $ 0.17 | 0.56 | 0.72
Cumulative impact on net income (loss) as a result of theft | $ 35.2 |  |
Parent Company [Member] |  |  |
Description Of Business [Line Items] |  |  |
Dividends declared per share to BGC Class A and Class B common stockholders | $ 0.17 | 0.56 | 0.72
Dividends paid per share to BGC Class A and Class B common stockholders | $ 0.17 | $ 0.56 | $ 0.72
Cumulative impact on net income (loss) as a result of theft | $ 35.2 |  |

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