Year when the debt instrument is scheduled to be fully repaid.

No definition available.

Name:	bgcp_DebtInstrumentMaturityYear
Namespace Prefix:	bgcp_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt instrument settlement date.

+ References

No definition available.

+ Details

Name:	bgcp_DebtInstrumentSettlementDate
Namespace Prefix:	bgcp_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Tendered amount on cash tender offer.

+ References

No definition available.

+ Details

Name:	bgcp_TenderedAmountOnCashTenderOffer
Namespace Prefix:	bgcp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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

- Definition

Description of the frequency of periodic payments (monthly, quarterly, annual).

+ References

+ Details

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

- Definition

The average effective interest rate during the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateDuringPeriod
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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=120391452&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=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the aggregate of total long-term debt, including current maturities and short-term debt.

+ References

No definition available.

+ Details

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

- Definition

Amount, before accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28555-108399

+ Details

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

- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

+ Details

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

- Definition

Aggregate amount of interest paid or due on all long-term debt.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.8)
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

+ Details

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

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: 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(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

+ Details

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

- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

The cash inflow from a borrowing supported by a written promise to pay an obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

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

- Definition

The cash outflow for a borrowing supported by a written promise to pay an obligation.

+ 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 (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3291-108585

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	srt_ConsolidatedEntitiesAxis=srt_ParentCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=bgcp_NewSeniorRevolvingCreditAgreementMember
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:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=bgcp_NewmarkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=bgcp_CashTenderOfferMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Schedule I - Parent Company Only - Long Term Debt - Additional Information (Detail) - USD ($)				12 Months Ended
	Jul. 10, 2020	Dec. 11, 2019	Sep. 27, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Aug. 11, 2020	Aug. 05, 2020	Nov. 28, 2018	Jul. 24, 2018	Sep. 08, 2017	May 27, 2016
Debt Instrument [Line Items]
Carrying amount of debt component				$ 1,319,784,000	$ 1,147,649,000
Unsecured Credit Agreement September 8, 2017 [Member]
Debt Instrument [Line Items]
Repayment of term loan						$ 195,000,000.0
Line of credit facility, outstanding amount						0
5.125% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Principal amount of notes												$ 300,000,000.0
Stated interest rate												5.125%
5.375% Senior Notes [Member]
Debt Instrument [Line Items]
Interest expense recorded				$ 25,600,000	$ 25,600,000	11,000,000.0
Principal amount of notes										$ 450,000,000.0
Stated interest rate				5.375%	5.375%					5.375%
Debt instrument, periodic interest payment description				payable in cash on January 24 and July 24 of each year, commencing January 24, 2019. The 5.375% Senior Notes will mature on July 24, 2023.
Senior Notes maturity date				Jul. 24, 2023
Redemption price as percentage of principal amount				101.00%
Carrying amount of debt component				$ 446,600,000						$ 444,200,000
Debt issuance costs										5,800,000
3.750% Senior Notes Due October 1, 2024 [Member]
Debt Instrument [Line Items]
Principal amount of notes			$ 300,000,000.0
Stated interest rate			3.75%
Parent Company [Member] \| Unsecured Credit Agreement September 8, 2017 [Member] \| Senior Revolving Credit and Converted Term Loan Agreement [Member]
Debt Instrument [Line Items]
Maximum revolving credit											$ 400,000,000.0
Credit agreement maturity date				Sep. 08, 2019
Interest expense recorded				$ 0	$ 0	1,000,000.0
Parent Company [Member] \| Unsecured Credit Agreement September 8, 2017 [Member] \| New Senior Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Credit agreement maturity date		Feb. 26, 2021		Nov. 28, 2020
Repayment of term loan						195,000,000.0
Line of credit facility, outstanding amount				$ 0	68,900,000	0
Interest expense recorded				$ 5,300	10,000,000.0	300,000
Deferred financing costs					$ 1,100,000
Average interest rate				2.88%	3.88%
Parent Company [Member] \| Unsecured Credit Agreement September 8, 2017 [Member] \| New Senior Revolving Credit Agreement [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Maximum revolving credit											$ 400,000,000.0
Parent Company [Member] \| Unsecured Credit Agreement September 8, 2017 [Member] \| New Senior Revolving Credit Agreement [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Maximum revolving credit									$ 350,000,000.0
Parent Company [Member] \| Unsecured Credit Agreement September 8, 2017 [Member] \| New Senior Revolving Credit Agreement [Member] \| Newmark [Member]
Debt Instrument [Line Items]
Proceeds from notes payable						195,000,000.0
Parent Company [Member] \| 5.125% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Interest expense recorded				$ 16,300,000	$ 16,200,000	16,200,000
Principal amount of notes												$ 300,000,000.0
Debt maturity year				2021
Stated interest rate				5.125%
Debt instrument, periodic interest payment description				payable in cash on May 27 and November 27 of each year, commencing November 27, 2016
Senior Notes maturity date				May 27, 2021
Redemption price as percentage of principal amount				101.00%
Carrying amount of debt component				$ 255,600,000								295,800,000
Debt issuance costs												$ 4,200,000
Parent Company [Member] \| 5.125% Senior Notes Due 2021 [Member] \| Cash Tender Offer [Member]
Debt Instrument [Line Items]
Principal amount of notes								$ 300,000,000.0
Stated interest rate								5.125%
Tendered amount on cash tender offer							$ 44,000,000.0
Debt instrument settlement date				Aug. 14, 2020
Parent Company [Member] \| 5.375% Senior Notes [Member]
Debt Instrument [Line Items]
Interest expense recorded				$ 25,500	25,600,000	11,000,000.0
Principal amount of notes										450,000,000.0
Stated interest rate				5.375%
Debt instrument, periodic interest payment description				payable in cash on January 24 and July 24 of each year, commencing January 24, 2019. The 5.375% Senior Notes will mature on July 24, 2023
Senior Notes maturity date				Jul. 24, 2023
Redemption price as percentage of principal amount				101.00%
Carrying amount of debt component				$ 446,600,000						444,200,000
Debt issuance costs										$ 5,800,000
Parent Company [Member] \| 3.750% Senior Notes Due October 1, 2024 [Member]
Debt Instrument [Line Items]
Interest expense recorded				$ 12,100,000	3,200,000	0
Principal amount of notes			$ 300,000,000.0
Stated interest rate				3.75%
Debt instrument, periodic interest payment description				payable on each April 1 and October 1, commencing April 1, 2020
Senior Notes maturity date				Oct. 01, 2024
Redemption price as percentage of principal amount			101.00%
Carrying amount of debt component			$ 296,100,000	$ 296,900,000
Debt issuance costs			$ 3,900,000
Parent Company [Member] \| 4.375% Senior Notes [Member]
Debt Instrument [Line Items]
Interest expense recorded				$ 6,500,000	$ 0	$ 0
Principal amount of notes	$ 300,000,000.0
Stated interest rate				4.375%
Debt instrument, periodic interest payment description				payable in cash on June 15 and December 15 of each year, commencing December 15, 2020.
Senior Notes maturity date				Dec. 15, 2025
Redemption price as percentage of principal amount	101.00%
Carrying amount of debt component	$ 296,800,000			$ 297,000,000.0
Debt issuance costs	$ 3,200,000