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Compensation - Activity Associated with LPU's Held by BGC Employees (Detail) - LPUs [Member] - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding at beginning of period	102,407,000	79,729,000	58,323,000
Number of Units, Granted	50,269,000	47,916,000	36,821,000
Number of Units, Redeemed/exchanged units	(14,642,000)	(21,110,000)	(14,908,000)
Number of Units, Forfeited units	(382,000)	(4,128,000)	(507,000)
Number of Units, Balance outstanding at end of period	137,652,000	102,407,000	79,729,000
Newmark Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding at beginning of period	14,607,000	22,113,000	26,510,000
Number of Units, Granted		662,000	2,694,000
Number of Units, Redeemed/exchanged units	(1,300,000)	(1,024,000)	(7,032,000)
Number of Units, Forfeited units	(105,000)	(7,144,000)	(59,000)
Number of Units, Balance outstanding at end of period	13,202,000	14,607,000	22,113,000

X

- Definition

Number of exchangeable limited partnership units redeemed or exchanged into the Company's class A common stock during the period.

+ References

No definition available.

+ Details

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X

- 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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X

- Definition

Number of shares under non-option equity instrument agreements that were cancelled as a result of occurrence of a terminating event.

+ 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)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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X

- Definition

Net number of non-option equity instruments granted to participants.

+ 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)(iv)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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X

- 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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