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Compensation - Activity Associated with Limited Partnership Units Awarded to BGC Employees (Detail) - Limited Partnership Units [Member]	3 Months Ended
	Mar. 31, 2019 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding at beginning of period	79,729,188
Number of Units, Granted	9,290,951
Number of Units, Redeemed/exchanged units	(4,073,748)
Number of Units, Forfeited units/other	(4,002,864)
Number of Units, Balance outstanding at end of period	80,943,527
Newmark Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding at beginning of period	22,113,105
Number of Units, Granted	508,522
Number of Units, Redeemed/exchanged units	(37,567)
Number of Units, Forfeited units/other	(7,095,471)
Number of Units, Balance outstanding at end of period	15,488,589

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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Balance Type:	na
Period Type:	duration

X

- Definition

Share based compensation arrangement by share based payment award non option equity instruments forfeitures and other.

+ References

No definition available.

+ Details

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Namespace Prefix:	bgcp_
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Balance Type:	na
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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.

+ Details

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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=109197908&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=109197908&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=109197908&loc=d3e5070-113901

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- Details

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- Details

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