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Summary of Significant Accounting Policies - Additional Information (Detail)
 3 Months Ended 11 Months Ended 12 Months Ended
Dec. 31, 2020
USD ($)
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
USD ($)
Nov. 30, 2018
Segment
Dec. 31, 2020
USD ($)
Segment
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
Significant Accounting Policies [Line Items]                    
Number of reportable segments | Segment           2 1      
Maturity days for highly liquid investments             The Company considers all highly liquid investments with maturities of 90 days or less at the date of acquisition that are not segregated under regulatory requirements, other than those used for trading purposes, to be cash equivalents.      
Allowance for doubtful accounts on accrued commissions receivable $ 14,000,000.0       $ 10,500,000   $ 14,000,000.0 $ 10,500,000    
Ownership interest             The Company’s policy is to consolidate all entities of which it owns more than 50% unless it does not have control over the entity.      
Impairment of goodwill $ 0       $ 0   $ 0 0 $ 0  
Impairment of goodwill or indefinite-lived intangible assets             $ 0 $ 0 $ 0  
US federal corporate tax rate             21.00% 21.00%   35.00%
Restricted stock saleable period Description             shares of restricted stock are generally saleable by partners in five to ten years.      
Payout period for post-termination awards             four equal yearly installments      
Limited partnership units exchangeability into Class A common stock             Following the Spin-Off, certain limited partnership units in BGC Holdings are granted exchangeability or redeemed in connection with the grant of shares of BGC Class A common stock on a one-for-one basis (subject to adjustment), and certain limited partnership units in Newmark Holdings are granted exchangeability or redeemed in connection with the grant of shares of Newmark Class A common stock based on the exchange ratio at the time.      
Percentage to preferred units 0.6875% 0.6875% 0.6875% 0.6875%     2.75%      
U.S. Federal [Member]                    
Significant Accounting Policies [Line Items]                    
Open tax year             2008      
U.S. State and Local [Member]                    
Significant Accounting Policies [Line Items]                    
Open tax year             2009      
Non-U.S [Member]                    
Significant Accounting Policies [Line Items]                    
Open tax year             2012      
Software, Including Software Development Costs [Member]                    
Significant Accounting Policies [Line Items]                    
Software amortization period             3 years      
Maximum [Member]                    
Significant Accounting Policies [Line Items]                    
Restricted stock saleable period             10 years      
Maximum [Member] | Computer and Communications Equipment [Member]                    
Significant Accounting Policies [Line Items]                    
Useful life of computer software             5 years      
Minimum [Member]                    
Significant Accounting Policies [Line Items]                    
Restricted stock saleable period             5 years      
Minimum [Member] | Computer and Communications Equipment [Member]                    
Significant Accounting Policies [Line Items]                    
Useful life of computer software             3 years