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Compensation - Summary of the BGC Holdings and Newmark Holdings LPUs held by BGC Employees (Detail) - LPUs [Member] - shares | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of Units, Balance outstanding | 137,652,000 | 102,407,000 | 79,729,000 | 58,323,000
Newmark [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of Units, Balance outstanding | 13,202,000 | 14,607,000 | 22,113,000 | 26,510,000
Regular Units [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of Units, Balance outstanding | 92,296,000 |  |  |
Regular Units [Member] | Newmark [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of Units, Balance outstanding | 9,480,000 |  |  |
Preferred Units [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of Units, Balance outstanding | 45,356,000 |  |  |
Preferred Units [Member] | Newmark [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of Units, Balance outstanding | 3,722,000 |  |  |

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