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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
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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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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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=120381028&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=120381028&loc=d3e5070-113901

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