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Compensation - Activity Associated with Limited Partnership Units (Detail) - shares
3 Months Ended 12 Months Ended
Mar. 31, 2021
Dec. 31, 2020
NEWMARK Group Inc Parent    
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding    
Beginning balance (in shares) 66,626,185 60,800,852
Number of Units, Issued (in shares) 1,115,181 12,569,298
Number of units, Redeemed/exchanged units (in shares) (3,450,288) (6,660,984)
Number of Units, Forfeited units (in shares) (158,167) (82,981)
Ending balance (in shares) 64,132,911 66,626,185
Exchangeable Units (units) 9,464,391 9,906,763
BGC Holdings, L.P.    
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding    
Beginning balance (in shares) 54,422,002 56,053,701
Number of Units, Issued (in shares) 159,057 1,071,612
Number of units, Redeemed/exchanged units (in shares) (1,989,419) (2,657,901)
Number of Units, Forfeited units (in shares) (848,335) (45,410)
Ending balance (in shares) 51,743,305 54,422,002
Exchangeable Units (units) 23,387,428 24,863,107
Regular units | NEWMARK Group Inc Parent    
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding    
Ending balance (in shares) 59,897,905  
Regular units | BGC Holdings, L.P.    
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding    
Ending balance (in shares) 50,291,393  
Preferred Units | NEWMARK Group Inc Parent    
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding    
Ending balance (in shares) 4,235,006  
Preferred Units | BGC Holdings, L.P.    
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding    
Ending balance (in shares) 1,451,912