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Financial Guarantee Liability - Provisions for Risk Sharing (Detail) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Guarantees [Abstract] |  |
Increase (decrease) to financial guarantee liability | $ 7 | $ (5)
Decrease (increase) to credit enhancement asset | 10 | 4
Total provision for risk-sharing obligations | $ 17 | $ (1)

X
- Definition Decrease (increase) to credit enhancement asset. + References No definition available. + Details
Name: | bgcp_DecreaseIncreaseToCreditEnhancementAsset
Namespace Prefix: | bgcp_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Increase (decrease) to financial guarantee liability. + References No definition available. + Details
Name: | bgcp_IncreaseDecreaseToFinancialGuaranteeLiability
Namespace Prefix: | bgcp_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Provision for risk-sharing obligations. + References No definition available. + Details
Name: | bgcp_ProvisionForRiskSharingObligations
Namespace Prefix: | bgcp_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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