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Compensation - Summary of Outstanding LPUs Held by BGC Employees with Stated Vesting (Detail) - LPUs [Member] - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Aggregate estimated grant date fair value $ 205,360 $ 138,324
BGC Holdings LPUs [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Aggregate estimated grant date fair value 43,684 30,699
Newmark Holdings LPUs [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Aggregate estimated grant date fair value $ 438 $ 1,171