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Limited Partnership Interests in BGC Holdings and Newmark Holdings - Additional Information (Detail) shares in Millions |  | 6 Months Ended
--- | --- | ---
Nov. 23, 2018 shares | Jun. 30, 2021 partnership installment shares
Related Party Transaction [Line Items] |  |
Number of operating partnerships | partnership |  | 2
Number Of Installments | installment |  | 4
LPUs |  |
Related Party Transaction [Line Items] |  |
Limited partnership interests exchange ratio |  | 0.9403
Newmark Holdings |  |
Related Party Transaction [Line Items] |  |
Calculation of contribution ratio, denominator |  | 2.2
Limited partnership interest, conversion ratio |  | 1
Percentage of preferred partnership units awarded |  | 0.6875%
Percentage of preferred partnership units awarded per calendar year |  | 2.75%
BGC Holdings |  |
Related Party Transaction [Line Items] |  |
Limited partnership interest, conversion ratio |  | 1
Percentage of preferred partnership units awarded |  | 0.6875%
Percentage of preferred partnership units awarded per calendar year |  | 2.75%
Cantor Rights to Purchase Exchangeable Units |  |
Related Party Transaction [Line Items] |  |
Right to exchange from Class A to Class B common stock (up to) (in shares) | shares | 10.3 | 23.6

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