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Limited Partnership Interests - Additional Information (Detail) - USD ($) shares in Millions, $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Jun. 30, 2019 | Dec. 31, 2019 | Nov. 30, 2018 | Sep. 26, 2018 | Jun. 18, 2018
Noncontrolling Interest [Abstract] |  |  |  |  |  |  |  |  |
Payout period for post-termination awards |  |  |  |  | four equal yearly installments |  |  |  |
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction |  |  | 94.95% |  | 94.95% |  |  |  |
Percentage to preferred units |  |  | 0.6875% | 0.6875% |  |  |  |  |
BGC Holdings, L.P. [Member] |  |  |  |  |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |  |  |  |  |
Limited partnership interest initial contribution ratio |  |  |  |  | 45.45% |  |  |  |
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction |  |  |  |  |  |  | 46.3895% |  |
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 |  |  |  |  | 23.0 |  |  |  |
Cantor Rights to Purchase Exchangeable Units [Member] |  |  |  |  |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |  |  |  |  |
Number of shares, right to exchange from Class A to Class A or Class B common stock |  |  |  |  | 23.0 |  |  |  |
Scenario, Forecast [Member] |  |  |  |  |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |  |  |  |  |
Percentage to preferred units | 0.6875% | 0.6875% |  |  |  | 2.75% |  |  |
Class A Common Stock [Member] |  |  |  |  |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |  |  |  |  |
Limited partnership units exchange ratio |  |  |  |  | 0.9495 |  |  |  |
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction |  |  |  |  |  |  | 46.3895% |  |
Newmark OpCo [Member] | RBC [Member] |  |  |  |  |  |  |  |  |
Noncontrolling Interest [Abstract] |  |  |  |  |  |  |  |  |
Exchangeable preferred limited partnership units issued |  |  | $ 325.0 |  | $ 325.0 |  |  | $ 150.0 | $ 175.0
Exchangeable preferred limited partnership units |  |  |  |  | $ 325.0 |  |  |  |

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