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Compensation - Activity Associated with Limited Partnership Units Awarded to BGC Employees (Detail) - LPUs [Member] - shares shares in Thousands	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, Redeemed	17,782	17,709	21,343
BGC Holdings LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Redeemed	16,618	17,209	18,227
Newmark Holdings LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Redeemed	1,164	500	3,116

X

- Definition

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

+ References

No definition available.

+ Details

Name:	bgcp_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentRedeemed
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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_PartnerTypeOfPartnersCapitalAccountAxis=bgcp_LimitedPartnershipUnitsMember
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
Balance Type:
Period Type: