Line of credit facility capacity available for specific purpose other than for trade purchases increased borrowing amount.

No definition available.

+ Details

Name:	bgcp_LineOfCreditFacilityCapacityAvailableForSpecificPurposeOtherThanForTradePurchasesIncreasedBorrowingAmount
Namespace Prefix:	bgcp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-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_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The maximum amount of borrowing capacity under a line of credit that is available as of the balance sheet date for a specific purpose other than for financing goods acquired for inventory or imminent delivery to a customer.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityCapacityAvailableForSpecificPurposeOtherThanForTradePurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_SecuritiesFinancingTransactionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_CreditFacilityAxis=bgcp_CommittedLinesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=bgcp_WarehouseLineDueOctoberElevenTwoThousandAndEighteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=bgcp_WarehouseLineDueSeptemberTwentyFiveTwoThousandEighteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=bgcp_WarehouseLineDueAugustTwentyTwoThousandEighteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=bgcp_WarehouseLineDueJuneTwentyTwoThousandEighteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collateralized Transactions - Schedule of Company Lines Available and Borrowings Outstanding (Parenthetical) (Detail) - USD ($) $ in Thousands			1 Months Ended	6 Months Ended	12 Months Ended
	Jul. 19, 2018	Jun. 08, 2018	Aug. 16, 2018	Jun. 30, 2018	Dec. 31, 2017
Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available				$ 950,000	$ 950,000
Warehouse Line Due October 11, 2018 [Member]
Securities Financing Transaction [Line Items]
Maturity date				Oct. 11, 2018	Oct. 11, 2018
Warehouse Line Due October 11, 2018 [Member] \| Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available				$ 300,000	$ 300,000
Warehouse Line Due October 11, 2018 [Member] \| Commited Lines [Member] \| Scenario Forecast [Member]
Securities Financing Transaction [Line Items]
Lines available, increased amount			$ 300,000
Lines available			$ 600,000
Warehouse Line Due September 25, 2018 [Member]
Securities Financing Transaction [Line Items]
Maturity date				Sep. 25, 2018	Sep. 25, 2018
Warehouse Line Due September 25, 2018 [Member] \| Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available				$ 200,000	$ 200,000
Warehouse Line Due August 20, 2018 [Member]
Securities Financing Transaction [Line Items]
Maturity date		Aug. 20, 2018		Aug. 20, 2018
Warehouse Line Due August 20, 2018 [Member] \| Subsequent Event [Member]
Securities Financing Transaction [Line Items]
Maturity date	Jun. 19, 2019
Warehouse Line Due August 20, 2018 [Member] \| Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available				$ 450,000
Warehouse Line Due June 20, 2018 [Member]
Securities Financing Transaction [Line Items]
Maturity date					Jun. 20, 2018
Warehouse Line Due June 20, 2018 [Member] \| Commited Lines [Member]
Securities Financing Transaction [Line Items]
Lines available					$ 450,000