Financial Guarantee Liability - Provisions for Risk Sharing (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Guarantees [Abstract]
Increase (decrease) to financial guarantee liability	$ 4	$ (134)	$ (3)	$ (347)
Decrease (increase) to credit enhancement asset		1	10	145
Increase (decrease) to contingent liability	(1)			6
Total provision for risk-sharing obligations	$ 3	$ (133)	$ 7	$ (196)

Increase (decrease) to financial guarantee liability.

No definition available.

Name:	bgcp_IncreaseDecreaseToFinancialGuaranteeLiability
Namespace Prefix:	bgcp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration