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Loans Held for Sale, at Fair Value - Summary of Loans Held for Sale at Cost Basis and Fair Value (Detail) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Loans Receivable Held For Sale Net [Abstract] |  |
Cost basis | $ 950,514 | $ 360,440
Fair Value | $ 965,639 | $ 362,635

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- Definition Loans held for sale cost basis. + References No definition available. + Details
Name: | bgcp_LoansHeldForSaleCostBasis
Namespace Prefix: | bgcp_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables. + References No definition available. + Details
Name: | us-gaap_LoansHeldForSaleFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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