Mortgage Servicing Rights, Net - Additional Information (Detail) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Mortgage Servicing Rights [Line Items]
Primary servicing portfolio	$ 55,600.0	$ 54,200.0
Special servicing portfolio	$ 3,400.0	$ 3,800.0
Minimum [Member] \| Measurement Input, Discount Rate [Member]
Mortgage Servicing Rights [Line Items]
Discount rate	3.0	3.0
Maximum [Member] \| Measurement Input, Discount Rate [Member]
Mortgage Servicing Rights [Line Items]
Discount rate	13.5	13.5
Discount Rate One [Member]
Mortgage Servicing Rights [Line Items]
Increase in discount rate	1.00%	1.00%
Decrease in fair value of servicing rights	$ (12.2)	$ (11.8)
Discount Rate Two [Member]
Mortgage Servicing Rights [Line Items]
Increase in discount rate	2.00%	2.00%
Decrease in fair value of servicing rights	$ (23.8)	$ (23.0)
Mortgage Servicing Rights [Member]
Mortgage Servicing Rights [Line Items]
Estimated fair value of MSRs	$ 435.1	$ 418.1

Mortgage servicing rights.

No definition available.

Name:	bgcp_MortgageServicingRightsLineItems
Namespace Prefix:	bgcp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of increase in discount rate of servicing rights.

+ References

No definition available.

+ Details

Name:	bgcp_PercentageOfIncreaseInDiscountRateOfServicingRights
Namespace Prefix:	bgcp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Primary servicing portfolio.

+ References

No definition available.

+ Details

Name:	bgcp_PrimaryServicingPortfolio
Namespace Prefix:	bgcp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Special servicing portfolio.

+ References

No definition available.

+ Details

Name:	bgcp_SpecialServicingPortfolio
Namespace Prefix:	bgcp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Value of input used to measure alternative investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_AlternativeInvestmentMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

+ Details

Name:	us-gaap_ServicingAssetAtFairValueAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The increase (decrease) in the fair value of servicing assets that are subsequently measured at fair value.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

+ Details

Name:	us-gaap_ServicingAssetAtFairValuePeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=bgcp_DiscountRateOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=bgcp_DiscountRateTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=bgcp_MortgageServicingRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: