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Compensation - Activity Associated with Limited Partnership Units Awarded to BGC Employees (Detail) - LPUs [Member] - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Redeemed	718	4,822	5,482	10,985
BGC Holdings LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Redeemed	614	4,694	5,087	10,515
Newmark Holdings LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Redeemed	104	128	395	470

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- Definition

Share-based compensation arrangement by share based payment award non option equity instrument redeemed.

+ References

No definition available.

+ Details

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Namespace Prefix:	bgcp_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

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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Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=bgcp_BGCHoldingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=bgcp_NewmarkHoldingsMember
Namespace Prefix:
Data Type:	na
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Period Type: