Debt instrument maturity month and year.

No definition available.

Name:	bgcp_DebtInstrumentMaturityMonthAndYear
Namespace Prefix:	bgcp_
Data Type:	xbrli:gYearMonthItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Debt instrument penalty interest.

+ References

No definition available.

+ Details

Name:	bgcp_DebtInstrumentPenaltyInterest
Namespace Prefix:	bgcp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fixed assets pledged for secured loan value.

+ References

No definition available.

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

- Definition

Line of credit facility maximum borrowing capacity option to increase.

+ References

No definition available.

+ Details

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

- Definition

Line of credit facility maximum borrowing capacity terminated outstanding amount.

+ References

No definition available.

+ Details

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

- Definition

Percentage of net cash proceedes of material asset sales.

+ References

No definition available.

+ Details

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

- Definition

Redemption price of debt instrument as a percentage of the principal amount.

+ References

No definition available.

+ Details

Name:	bgcp_SeniorNotesRedemptionPricePercentage
Namespace Prefix:	bgcp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Subsidiary ownership percentage.

+ References

No definition available.

+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Earliest date before maturity when the issuer can call the outstanding debt instrument for repayment or conversion to equity, in CCYY-MM-DD format.

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Data Type:	xbrli:dateItemType
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Period Type:	duration

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Balance Type:	credit
Period Type:	instant

- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

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

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Balance Type:	credit
Period Type:	instant

- Definition

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

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Data Type:	xbrli:stringItemType
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Period Type:	duration

- Definition

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

+ References

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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.

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

+ References

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Data Type:	xbrli:dateItemType
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

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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

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Period Type:	instant

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

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Amount of the cost of borrowed funds accounted for as interest expense.

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

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- 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.

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Balance Type:	credit
Period Type:	instant

- Definition

Amount of the fee for available but unused credit capacity under the credit facility.

+ References

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Balance Type:	debit
Period Type:	duration

- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

+ Details

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- Definition

Description of the terms of a credit facility arrangement. Terms typically include interest rate, collateral required, guarantees required, repayment requirements, and restrictions on use of assets and activities of the entity.

+ References

+ Details

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Period Type:	duration

- Definition

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

+ References

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Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The effective interest rate at the end of the reporting period.

+ References

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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Balance Type:	credit
Period Type:	instant

- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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Balance Type:	credit
Period Type:	instant

- Definition

Number of new stock issued during the period.

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Balance Type:	na
Period Type:	duration

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Name:	us-gaap_CreditFacilityAxis=bgcp_SeniorRevolvingCreditFacilityAndConvertedTermLoanAgreementMember
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Notes Payable, Other and Short-term Borrowings - Additional Information (Detail) shares in Millions								3 Months Ended		9 Months Ended
	Jun. 30, 2018	Dec. 31, 2017 USD ($)	Sep. 08, 2017 USD ($)	Aug. 23, 2017 USD ($)	Aug. 22, 2017 USD ($)	Feb. 25, 2016 USD ($)	Apr. 28, 2015 shares	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2017	Mar. 31, 2018 BRL (R$)	Aug. 23, 2017 BRL (R$)	Aug. 22, 2017 BRL (R$)	May 31, 2017 USD ($)	May 27, 2016 USD ($)	Mar. 13, 2015 USD ($)	Dec. 09, 2014 USD ($)	Jun. 26, 2012 USD ($)
Debt Instrument [Line Items]
Debt instrument, fair value		$ 992,250,000						$ 977,550,000
Redemption price as percentage of principal amount								100.00%
Carrying amount of debt component		$ 1,656,555,000						$ 1,381,960,000
Interest expense								27,138,000	$ 18,763,000
IPO [Member]
Debt Instrument [Line Items]
Net proceeds from the IPO to partially repay the term loan								$ 304,300,000
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
Credit agreement maturity date								Sep. 08, 2019
Debt instrument, basis point		0.50%
Line of credit facility, description								Borrowings under the converted term loan bear interest at either LIBOR or a defined base rate plus an additional margin which ranges from 50 basis points to 325 basis points depending on the Company’s debt rating as determined by S&P and Fitch and whether such loan is a LIBOR loan or a base rate loan. As there were amounts outstanding under the senior term loan as of December 31, 2017, the pricing increased by 50 basis points, and if there are any amounts outstanding under the senior term loan as of June 30, 2018, the pricing shall increase by an additional 75 basis points (125 basis points in the aggregate) until the senior term loan is paid in full. From and after the repayment in full of the senior term loan, the pricing shall return to the levels previously described.
Debt instrument gross amount								$ 400,000,000
Line of credit facility, outstanding amount								397,700,000
Deferred financing costs								2,300,000
Debt instrument, fair value		$ 400,000,000						$ 400,000,000
Current interest rate								3.99%			3.99%
Interest expense recorded								$ 4,600,000
Unsecured Credit Agreement September 8, 2017 [Member] \| Senior Revolving Credit and Converted Term Loan Agreement [Member] \| Scenario Forecast [Member]
Debt Instrument [Line Items]
Debt instrument, basis point	0.75%
Unsecured Credit Agreement September 8, 2017 [Member] \| Senior Revolving Credit and Converted Term Loan Agreement [Member] \| LIBOR or Defined Base Rate [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point			0.50%
Unsecured Credit Agreement September 8, 2017 [Member] \| Senior Revolving Credit and Converted Term Loan Agreement [Member] \| LIBOR or Defined Base Rate [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point			3.25%
Unsecured Credit Agreement September 8, 2017 [Member] \| Senior Term Loan Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum revolving credit			$ 575,000,000
Credit agreement maturity date								Sep. 08, 2019
Debt instrument, basis point		0.50%
Line of credit facility, description								Borrowings under the senior term loan bear interest at either LIBOR or a defined base rate plus an additional margin which ranges from 50 basis points to 325 basis points depending on the Company’s debt rating as determined by S&P and Fitch and whether such loan is a LIBOR loan or a base rate loan. As there were amounts outstanding under the senior term loan as of December 31, 2017, the pricing increased by 50 basis points, and if there are any amounts outstanding under the senior term loan as of June 30, 2018, the pricing shall increase by an additional 75 basis points (125 basis points in the aggregate) until the senior term loan is paid in full. From and after the repayment in full of the senior term loan, the pricing shall return to the levels previously described.
Line of credit facility, outstanding amount								$ 0
Debt instrument, fair value		$ 270,700
Interest expense recorded								2,600,000
Line of credit facility, outstanding amount								270,700,000
Unsecured Credit Agreement September 8, 2017 [Member] \| Senior Term Loan Credit Agreement [Member] \| Newmark [Member]
Debt Instrument [Line Items]
Percentage of net cash proceeds of all material asset sales			100.00%
Unsecured Credit Agreement September 8, 2017 [Member] \| Senior Term Loan Credit Agreement [Member] \| Scenario Forecast [Member]
Debt Instrument [Line Items]
Debt instrument, basis point	0.75%
Unsecured Credit Agreement September 8, 2017 [Member] \| Senior Term Loan Credit Agreement [Member] \| LIBOR or Defined Base Rate [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point			0.50%
Unsecured Credit Agreement September 8, 2017 [Member] \| Senior Term Loan Credit Agreement [Member] \| LIBOR or Defined Base Rate [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point			3.25%
8.125% Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, fair value		$ 116,550,000						115,650,000
Interest expense recorded								$ 2,300,000	2,300,000
Stated interest rate		8.125%						8.125%			8.125%
Principal amount of notes																		$ 112,500,000
Debt maturity year								2042
Earliest call date								Jun. 26, 2017
Carrying amount of debt component																		108,700,000
Debt issuance costs																		$ 3,800,000
8.125% Senior Notes [Member] \| Level 1 [Member]
Debt Instrument [Line Items]
Stated interest rate								8.125%			8.125%
5.375% Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, fair value		$ 313,125,000						$ 309,750,000
Interest expense recorded								$ 4,300,000	4,300,000
Stated interest rate		5.375%						5.375%			5.375%
Principal amount of notes																	$ 300,000,000
Debt maturity year								2019
Redemption price as percentage of principal amount								101.00%
Carrying amount of debt component																	295,100,000
Debt issuance costs																	$ 4,900,000
Debt instrument, periodic interest payment description								payable in cash on June 9 and December 9 of each year, commencing June 9, 2015.
Senior Notes maturity date								Dec. 09, 2019
5.375% Senior Notes [Member] \| Level 2 [Member]
Debt Instrument [Line Items]
Stated interest rate								5.375%			5.375%
8.375% Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, fair value		$ 247,200,000						$ 242,400,000
Interest expense recorded								$ 5,000,000	5,000,000
Stated interest rate		8.375%						8.375%			8.375%
Principal amount of notes								$ 240,000,000
Debt instrument, periodic interest payment description								semi-annually
Debt maturity date								2018-07
Senior notes, carrying value								$ 241,300,000
Debt instrument penalty interest								2.00%
8.375% Senior Notes [Member] \| Level 2 [Member]
Debt Instrument [Line Items]
Stated interest rate								8.375%			8.375%
8.375% Senior Notes [Member] \| GFI Group Inc. [Member]
Debt Instrument [Line Items]
Stated interest rate								8.375%			8.375%
Debt instrument penalty interest								0.25%
Stock issued during period, shares, new issues \| shares							43.0
Percentage of outstanding common stock owned by parent							67.00%
5.125% Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, fair value		$ 315,375,000						$ 309,750,000
Interest expense recorded								$ 4,000,000	4,000,000
Stated interest rate		5.125%						5.125%			5.125%				5.125%
Principal amount of notes															$ 300,000,000
Debt maturity year								2021
Redemption price as percentage of principal amount								101.00%
Carrying amount of debt component															295,800,000
Debt issuance costs															$ 4,200,000
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
5.125% Senior Notes [Member] \| Level 2 [Member]
Debt Instrument [Line Items]
Stated interest rate								5.125%			5.125%
3.70% [Member]
Debt Instrument [Line Items]
Outstanding balance of the secured loan arrangements								$ 7,400,000
3.70% [Member] \| Secured Loan Arrangements [Member]
Debt Instrument [Line Items]
Debt instrument gross amount																$ 28,200,000
Deferred financing costs								$ 100,000
Stated interest rate																3.70%
Senior Notes maturity date								Mar. 13, 2019
Value of the fixed assets pledged								$ 200,000
Interest expense								100,000	200,000
3.44% [Member]
Debt Instrument [Line Items]
Outstanding balance of the secured loan arrangements								$ 24,600,000
3.44% [Member] \| Secured Loan Arrangements [Member]
Debt Instrument [Line Items]
Debt instrument gross amount														$ 29,900,000
Stated interest rate														3.44%
Senior Notes maturity date								May 31, 2021
Value of the fixed assets pledged								$ 14,300,000
Interest expense								200,000
Unsecured Credit Agreement [Member] \| Intra-Day Overdraft Credit Line [Member]
Debt Instrument [Line Items]
Maximum revolving credit				$ 15,100,000								R$ 50,000,000
Credit agreement maturity date				Jun. 14, 2018
Line of credit facility, outstanding amount								0
Line of credit facility, fee percentage				1.00%
Line of credit facility, fee								$ 39,000
Unsecured Credit Agreement [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum revolving credit						$ 150,000,000
Credit agreement maturity date						Sep. 08, 2017
Line of credit facility, description								Borrowings under this facility bear interest at either LIBOR or a defined base rate plus an additional margin which ranges from 50 basis points to 250 basis points depending on the Company’s debt rating as determined by S&P and Fitch and whether such loan is a LIBOR loan or a base rate loan.
Line of credit facility, outstanding amount		$ 0						$ 0
Interest expense recorded									$ 200,000
Maximum revolving credit, option to increase						$ 200,000,000
Line of credit facility, terminated outstanding amount			$ 150,000,000
Debt instrument, description of reference rate										LIBOR
Unsecured Credit Agreement [Member] \| Revolving Credit Facility [Member] \| LIBOR or Defined Base Rate [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point						0.50%
Unsecured Credit Agreement [Member] \| Revolving Credit Facility [Member] \| LIBOR or Defined Base Rate [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, basis point						2.50%
Unsecured Loan Agreement [Member] \| Short Term Loans [Member]
Debt Instrument [Line Items]
Maximum revolving credit					$ 6,000,000								R$ 20,000,000
Credit agreement maturity date					May 19, 2018
Line of credit facility, description								Borrowings under this facility bear interest at the Brazilian Interbank offering rate plus 3.30%.
Line of credit facility, outstanding amount								$ 6,000,000			R$ 19,800,000
Current interest rate								9.75%			9.75%
Interest expense recorded								$ 100,000
Debt instrument, description of reference rate								Brazilian Interbank offering rate
Unsecured Loan Agreement [Member] \| Brazilian Interbank Offering Rate [Member] \| Short Term Loans [Member]
Debt Instrument [Line Items]
Debt instrument, basis point					3.30%