Compensation - Activity Associated with Limited Partnership Units (Detail) - Limited Partnership Units [Member]	3 Months Ended
Mar. 31, 2018 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding at beginning of period	29,413,143
Number of Units, Redeemed/exchanged units	(160,479)
Number of Units, Forfeited units	(8,288)
Number of Units, Balance outstanding at end of period	33,011,995
BGC Holdings, L.P. [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding at beginning of period	64,708,915
Number of Units, Redeemed/exchanged units	(353,054)
Number of Units, Forfeited units	(18,233)
Number of Units, Balance outstanding at end of period	64,337,628

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

No definition available.

+ Details

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Period Type:	duration

- 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.

+ Details

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- Definition

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

+ References

+ Details

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- Definition

Number of equity instruments other than options outstanding, including both vested and non-vested instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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=96867065&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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=96867065&loc=d3e5070-113901

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- Details

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- Details

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