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Derivatives - Offsetting of Derivative Instruments (Parenthetical) (Detail) - USD ($) | Jun. 30, 2020 | Dec. 31, 2019
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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

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- References No definition available. + Details
Name: | us-gaap_OffsettingAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration