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Limited Partnership Interests in Newmark Holdings and BGC Holdings (Details)
3 Months Ended 12 Months Ended
Mar. 31, 2021
tranche
shares
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2019
Dec. 13, 2017
Related Party Transaction              
Payout period for post-termination awards 4 years            
Percentage to preferred units (in percentage)   0.6875% 0.6875% 0.6875% 0.6875% 2.75%  
Non Distribution Earning Units              
Related Party Transaction              
Award vesting period (in years) 4 years            
Common Class A              
Related Party Transaction              
Limited partnership units exchange ratio (in percentage)             1
Common Class B | Cantor              
Related Party Transaction              
Exchangeable partnership units (in shares) 23,700,000            
Share converted (in shares) 1            
Newmark OpCo | RBC              
Related Party Transaction              
Exchangeable preferred limited partnership units, number of tranches | tranche 4            
NEWMARK Group Inc Parent | Common Class A              
Related Party Transaction              
Limited partnership units exchange ratio (in percentage) 0.9365