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Compensation - Activity Associated with Limited Partnership Units Awarded to BGC Employees (Detail) - LPUs [Member] - shares shares in Thousands | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Number of Units, Redeemed | 1,277 | 4,193
BGC Holdings LPUs [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Number of Units, Redeemed | 1,097 | 4,001
Newmark Holdings LPUs [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Number of Units, Redeemed | 180 | 192

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- Definition Share-based compensation arrangement by share based payment award non option equity instrument redeemed. + References No definition available. + Details
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Namespace Prefix: | bgcp_
Data Type: | xbrli:sharesItemType
Balance Type: | na
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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=bgcp_BGCHoldingsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=bgcp_NewmarkHoldingsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |