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Related Party Transactions - Other Related Party Transactions and Cantor Rights to Purchase Cantor Units from Newmark Holdings (Details)
3 Months Ended
Mar. 31, 2021
USD ($)
shares
Nov. 30, 2018
USD ($)
Mar. 31, 2021
USD ($)
Mar. 31, 2020
USD ($)
Dec. 31, 2020
USD ($)
May 15, 2020
USD ($)
Nov. 06, 2018
Related Party Transaction              
Accounts receivable, related parties $ 0   $ 0   $ 0    
Payables to related parties $ 6,045,000   6,045,000   $ 4,392,000    
Operating lease payments     11,800,000 $ 11,700,000      
Proceeds from sales of partnership interest     4,025,000 $ 0      
Common Stock              
Related Party Transaction              
Limited partnership units exchange ratio (in percentage)             1
Cantor | Founding partner interest              
Related Party Transaction              
Stock issued (in shares) | shares 273,088            
Proceeds from sales of partnership interest $ 1,105,598            
Cantor | Limited partner interest              
Related Party Transaction              
Stock issued (in shares) | shares 735,625            
Proceeds from sales of partnership interest $ 2,918,919            
Affiliated Persons or Entities | BGC Holdings, L.P.              
Related Party Transaction              
Accounts receivable, related parties           $ 1,100,000  
Lease, term of contract (in years)           1 year  
Affiliated Persons or Entities | BGC Holdings, L.P.              
Related Party Transaction              
Operating lease payments     $ 300,000        
Cantor Credit Agreement | Cantor              
Related Party Transaction              
Maximum revolving credit   $ 250,000,000          
Basis spread on variable rate (in percentage)   1.00%