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Compensation - Summary of Compensation Expense Related to Amortization of LPUs Held by BGC Employees (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
LPU amortization	$ 17,094	$ 16,309
Stated vesting schedule [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
LPU amortization	17,074	16,277
Post-termination payout [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
LPU amortization	$ 20	$ 32

X

- Definition

Share based compensation arrangement by share based payment award limited partners capital units amortization.

+ References

No definition available.

+ Details

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Namespace Prefix:	bgcp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- 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_PlanNameAxis=bgcp_StatedVestingScheduleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_PlanNameAxis=bgcp_PostTerminationPayoutMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: