Average redemption price per unit for founding/working partner units.

No definition available.

Name:	bgcp_FoundingWorkingPartnerUnitsAverageRedemptionPrice
Namespace Prefix:	bgcp_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Limited partnership units average redemption price per unit.

+ References

No definition available.

+ Details

Name:	bgcp_LimitedPartnershipUnitsAverageRedemptionPricePerUnit
Namespace Prefix:	bgcp_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

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

- Definition

Total change in each class of partners' capital accounts during the year due to redemptions and adjustments to redemption value. All partners include general, limited and preferred partners.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 5
-Subparagraph (SAB Topic 4.F)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187171-122770

+ Details

Name:	us-gaap_PartnersCapitalAccountRedemptions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The number of units redeemed during the year of each class of partners' capital account. Units represent shares of ownership of the general, limited, and preferred partners.

+ References

+ Details

Name:	us-gaap_PartnersCapitalAccountUnitsRedeemed
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash outflow to reacquire common stock during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585

+ Details

Name:	us-gaap_PaymentsForRepurchaseOfCommonStock
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Total cost of shares repurchased divided by the total number of shares repurchased.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

+ Details

Name:	us-gaap_TreasuryStockAcquiredAverageCostPerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares that have been repurchased during the period and are being held in treasury.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_TreasuryStockSharesAcquired
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PartnerTypeOfPartnersCapitalAccountAxis=bgcp_LimitedPartnershipMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PartnerTypeOfPartnersCapitalAccountAxis=us-gaap_OtherOwnershipInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Stock Transactions and Unit Redemptions - Unit Redemption and Share Repurchase Activity (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	May 01, 2018	Apr. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Class of Stock [Line Items]
Average price for redemption of limited partnership units				$ 13.10	$ 11.20	$ 13.46	$ 10.97
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired	6,543	281,339	438,657			726,539
Aggregate purchase price of Class A common stock						$ 9,711	$ 10,590
Average price of Class A common stock	$ 11.65	$ 13.12	$ 13.55			$ 13.37
Class A Common Stock [Member]
Class of Stock [Line Items]
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired				726,539	397,539	726,539	950,494	1,386,769
Aggregate purchase price of Class A common stock				$ 9,700	$ 4,500	$ 9,700	$ 10,600
Average price of Class A common stock				$ 13.37	$ 11.25	$ 13.37	$ 11.14
Limited Partnership [Member]
Class of Stock [Line Items]
Redeemed limited partnership units				4,200,000	1,600,000	5,900,000	4,600,000
Aggregate redemption price of limited partnership units				$ 55,300	$ 18,500	$ 80,000	$ 50,000
Other Ownership Interest [Member]
Class of Stock [Line Items]
Redeemed limited partnership units				100,000	300,000	100,000	300,000
Aggregate redemption price of limited partnership units				$ 1,000	$ 2,800	$ 1,100	$ 3,000
Average price for redemption of founding/working partner units				$ 13.09	$ 10.72	$ 13.13	$ 10.72