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Schedule I - Parent Company Only - Organization and Basis of Presentation - Additional Information (Detail) - $ / shares
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Description Of Business [Line Items]      
Dividends declared per share to BGC Class A and Class B common stockholders $ 0.56 $ 0.72 $ 0.70
Dividends paid per share to BGC Class A and Class B common stockholders 0.56 0.72 0.70
Parent Company [Member]      
Description Of Business [Line Items]      
Dividends declared per share to BGC Class A and Class B common stockholders 0.56 0.72 0.70
Dividends paid per share to BGC Class A and Class B common stockholders $ 0.56 $ 0.72 $ 0.70