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Derivatives - Offsetting of Derivative Instruments (Parenthetical) (Detail) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Offsetting [Abstract]
Additional balance in gross amounts not offset	$ 0	$ 0

X

- Definition

Derivative additional balance in gross amounts not offset.

+ References

No definition available.

+ Details

Name:	bgcp_DerivativeAdditionalBalanceInGrossAmountsNotOffset
Namespace Prefix:	bgcp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- References

No definition available.

+ Details

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