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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