Financial Instruments (Narrative) (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedges, Assets [Abstract]
Number of derivative require collateral security	$ 0	$ 0

Derivative Contracts Held that Require Collateral Securities

No definition available.

Name:	bgc_DerivativeContractsHeldthatRequireCollateralSecurities
Namespace Prefix:	bgc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant