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Limited Partnership Interest in Newmark Holdings - Additional Information (Detail) shares in Millions | 3 Months Ended | 12 Months Ended
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Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 Partnership shares | Dec. 31, 2018
Noncontrolling Interest [Abstract] |  |  |  |  |
Payout period for post-termination awards |  |  |  | Four equal yearly installments |
Percentage to preferred units |  |  |  | 0.6875% |
CF Group Management, Inc. [Member] |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |
Number of shares, right to exchange from Class A to Class A or Class B common stock |  |  |  | 34.6 |
Cantor Rights to Purchase Exchangeable Units [Member] | Maximum [Member] |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |
Number of shares, right to exchange from Class A to Class A or Class B common stock |  |  |  | 34.6 |
Scenario, Forecast [Member] |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |
Percentage to preferred units | 0.6875% | 0.6875% | 0.6875% |  | 2.75%
BGC Partners, Inc. [Member] |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |
Number of operating partnerships | Partnership |  |  |  | 2 |
BGC Holdings Limited Partnership [Member] |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |
Limited partnership interest initial contribution ratio |  |  |  | 45.45% |
Limited partnership interest contribution ratio |  |  |  | 47.02% |

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