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Compensation - Compensation Expense Related to Issuance of BGC or Newmark Class A Common Stock and Grants of Exchangeability on BGC Holdings and Newmark Holdings LPUs held by BGC Employees (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
LPUs [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Issuance of common stock and grants of exchangeability | $ 84,966 | $ 100,948 | $ 150,147

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- Definition Issuance of common stock and grants of exchangeability. + References No definition available. + Details
Name: | bgcp_IssuanceOfCommonStockAndGrantsOfExchangeability
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

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- Details
Name: | us-gaap_PartnerTypeOfPartnersCapitalAccountAxis=bgcp_LimitedPartnershipUnitsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |