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Organization and Basis of Presentation (Details)
$ in Thousands
 12 Months Ended
Nov. 30, 2020
USD ($)
shares
Dec. 02, 2019
USD ($)
shares
Sep. 26, 2018
USD ($)
tranche
shares
Jun. 28, 2013
USD ($)
shares
Dec. 31, 2018
USD ($)
Mar. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Nov. 30, 2019
USD ($)
Jun. 18, 2018
USD ($)
Description Of Business                    
Shares settled (in shares) | shares 741,505 898,685                
Investment owned shares settled fair value $ 93,500 $ 93,500                
Shares owned (in shares) | shares 250,742 93,562                
Marketable securities           $ 4,001 $ 33,283      
Assets           4,000,733 3,982,450      
Total liabilities           3,009,869 3,041,258      
Retained earnings           372,387 342,764      
Common Stock                    
Description Of Business                    
Fair value of equity security           $ 4,000 $ 33,300      
Cumulative Effect, Period of Adoption, Adjustment                    
Description Of Business                    
Assets               $ (8,000)    
Total liabilities               17,900    
Retained earnings               $ (25,900)    
Nasdaq Earn-Out | Common Stock                    
Description Of Business                    
Fair value of equity security $ 121,900                  
Marketable securities                 $ 98,600  
Nasdaq Omx                    
Description Of Business                    
Purchase consideration paid in cash       $ 750,000            
Period for expected payment under Common stock transaction (years)       15 years            
Nasdaq Omx | Maximum                    
Description Of Business                    
Shares received from transaction (in shares) | shares     992,247              
BGC Partners Inc | Nasdaq Omx | Maximum                    
Description Of Business                    
Expected payment of shares under common stock transaction (in shares) | shares       14,883,705            
BGC Partners Inc | Nasdaq Omx | Minimum                    
Description Of Business                    
Gross revenue on expected payment per year under common stock transaction       $ 25,000            
Newmark OpCo | RBC                    
Description Of Business                    
Exchangeable preferred limited partnership units issued (in shares)     $ 150,000             $ 175,000
Proceeds from issuance of EPUs         $ 266,100          
Exchangeable preferred limited partnership units, number of tranches | tranche     4