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Limited Partnership Interests in BGC Holdings and Newmark Holdings - Additional Information (Detail) - shares |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Nov. 23, 2018 | Jun. 05, 2015 | Dec. 31, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Sep. 30, 2018 | Dec. 31, 2020
Noncontrolling Interest [Abstract] |  |  |  |  |  |  |  |
Number of operating partnerships |  |  |  |  |  | 2 |  |
Limited partnership interest contribution ratio |  |  |  |  |  | 45.45% |  |
Limited partnership interests Exchange Ratio |  |  |  |  |  | 94.61% |  |
Payout period for post-termination awards |  |  |  |  |  | four equal yearly installments |  |
Percentage to preferred units |  |  |  |  |  | 0.6875% |  |
Distribution ratio |  |  |  |  |  | 46.3895% | 45.45% |
Common stock exchangeable basis |  |  |  |  |  | one-for-one basis |  |
Newmark [Member] |  |  |  |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |  |  |  |
Common stock exchangeable basis |  |  |  |  |  | one-for-one |  |
CF Group Management, Inc. [Member] |  |  |  |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |  |  |  |
Number of shares, right to exchange from Class A to Class B common stock | 712,907 | 34,649,693 |  |  |  | 23,600,000 |  |
Cantor Rights to Purchase Exchangeable Units [Member] |  |  |  |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |  |  |  |
Number of shares, right to exchange from Class A to Class B common stock | 10,323,366 |  |  |  |  |  |  |
Common stock exchangeable basis |  |  |  |  |  | one-for-one |  |
Cantor Rights to Purchase Exchangeable Units [Member] | Maximum [Member] |  |  |  |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |  |  |  |
Number of shares, right to exchange from Class A to Class B common stock |  | 34,649,693 |  |  |  | 23,600,000 |  |
Scenario Forecast [Member] |  |  |  |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |  |  |  |
Percentage to preferred units |  |  | 0.6875% | 0.6875% | 0.6875% |  |  | 2.75%

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- Definition Class Of Common Stock Exchanged To Another Class Of Common Stock + References No definition available. + Details
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- Definition Common stock exchangeable basis. + References No definition available. + Details
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- Definition Limited partnership interest contribution ratio. + References No definition available. + Details
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- Definition Limited partnership interest, distribution ratio. + References No definition available. + Details
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- Definition Limited partnership interests exchange ratio, subject to adjustment. + References No definition available. + Details
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- Definition Number of operating partnerships. + References No definition available. + Details
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- Definition Percentage of net profits of consolidated subsidiary allocated to preferred units. + References No definition available. + Details
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- Definition Payout period for post-termination awards. + References No definition available. + Details
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- References No definition available. + Details
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- Details
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- Details
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