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Compensation - Summary of the BGC Holdings and Newmark Holdings LPUs (Detail) - LPUs [Member] - shares | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | --- | ---
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of Units, Balance outstanding | 102,407,000 | 79,729,000 | 58,323,000 | 50,835,000
Newmark [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of Units, Balance outstanding | 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 | 70,831,000 |  |  |
Regular Units [Member] | Newmark [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of Units, Balance outstanding | 10,418,000 |  |  |
Preferred Units [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of Units, Balance outstanding | 31,576,000 |  |  |
Preferred Units [Member] | Newmark [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of Units, Balance outstanding | 4,189,000 |  |  |

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- Definition Number of equity instruments other than options outstanding, including both vested and non-vested instruments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(ii) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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