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

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.

+ References

No definition available.

+ Details

Name:	bgcp_LineOfCreditFacilityMaximumBorrowingCapacityTerminated
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 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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

Reference 2: 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

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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- 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.

+ Details

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

- 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=114775744&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=114775744&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=6877327&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=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=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=114775744&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=116637391&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=6877327&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=6876686&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=119400593&loc=d3e572229-122910

+ Details

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

- 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

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

- 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

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

- 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=118932676&loc=d3e3255-108585

+ Details

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

- Definition

Amount of cash outflow for payment of an obligation from a lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

+ 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=118932676&loc=d3e3291-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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

- Definition

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

+ References

+ 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_CreditFacilityAxis=bgcp_ConvertedTermLoanMember
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_CreditFacilityAxis=bgcp_NewSeniorRevolvingCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=bgcp_FivePointThreeSevenFivePercentSeniorNotesDueTwoThousandNinenteenMember
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_FivePointThreeSevenFivePercentSeniorNotesDueTwoThousandTwentyThreeMember
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:	us-gaap_DebtInstrumentAxis=bgcp_UnsecuredCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
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_LondonInterbankOfferedRateLIBOROrDefinedBaseRateMember
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:	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_SeniorTermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Schedule I - Parent Company Only - Long Term Debt - Additional Information (Detail) - USD ($)				12 Months Ended
	Dec. 11, 2019	Jul. 24, 2018	Feb. 25, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 27, 2019	Nov. 28, 2018	Sep. 08, 2017	May 27, 2016	Dec. 09, 2014	Jun. 26, 2012
Debt Instrument [Line Items]
Line of credit facility, outstanding amount				$ 0
Redemption price as percentage of principal amount				100.00%
Carrying amount of debt component				$ 1,147,649,000	$ 768,710,000
Unsecured Credit Agreement September 8, 2017 [Member]
Debt Instrument [Line Items]
Line of credit facility, outstanding amount					0
Repayment of term loan					195,000,000.0
Unsecured Credit Agreement September 8, 2017 [Member] \| Converted Term Loan [Member] \| Newmark [Member]
Debt Instrument [Line Items]
Line of credit facility, outstanding amount						$ 397,300,000
Repayments of revolving credit					397,300,000
Unsecured Credit Agreement September 8, 2017 [Member] \| New Senior Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum revolving credit								$ 350,000,000.0
Credit agreement maturity date				Nov. 28, 2020
Line of credit facility, outstanding amount				$ 68,900,000	0
Interest expense recorded				10,000,000.0	300,000	0
Deferred financing costs				$ 1,100,000
Average interest rate				3.88%
8.125% Senior Notes Due 2042 [Member]
Debt Instrument [Line Items]
Interest expense recorded				$ 0	$ 0	0
Principal amount of notes												$ 112,500,000
Stated interest rate				8.125%	8.125%
Debt maturity year				2042
Earliest call date				Jun. 26, 2017
Carrying amount of debt component												108,700,000
Debt issuance costs												3,800,000
5.375% Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Stated interest rate				5.375%
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 Due 2023 [Member]
Debt Instrument [Line Items]
Interest expense recorded				$ 25,600,000	$ 11,000,000.0	0
Principal amount of notes		$ 450,000,000.0
Stated interest rate		5.375%		5.375%	5.375%
Debt maturity year		2023
Redemption price as percentage of principal amount				101.00%
Carrying amount of debt component		$ 444,200,000
Debt issuance costs		5,800,000
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 due 2023 will mature on July 24, 2023.
Senior Notes maturity date				Jul. 24, 2023
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%
Unsecured Credit Agreement [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum revolving credit			$ 150,000,000.0
Credit agreement maturity date			Sep. 08, 2017
Line of credit facility, outstanding amount				$ 0	$ 0
Interest expense recorded						2,400,000
Maximum revolving credit terminated			$ 25,000,000.0
Debt instrument, description of reference rate				LIBOR
Line of credit facility, description				Borrowings under this facility bore 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.
Maximum revolving credit, option to increase			$ 200,000,000.0
Line of credit facility, terminated outstanding amount					25,000,000.0				$ 150,000,000.0
Unsecured Credit Agreement [Member] \| Revolving Credit Facility [Member] \| Maximum [Member] \| LIBOR or Defined Base Rate [Member]
Debt Instrument [Line Items]
Debt instrument, basis point			2.50%
Unsecured Credit Agreement [Member] \| Revolving Credit Facility [Member] \| Minimum [Member] \| LIBOR or Defined Base Rate [Member]
Debt Instrument [Line Items]
Debt instrument, basis point			0.50%
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	1,000,000.0	4,300,000
Parent Company [Member] \| Unsecured Credit Agreement September 8, 2017 [Member] \| Converted Term Loan [Member] \| Newmark [Member]
Debt Instrument [Line Items]
Line of credit facility, outstanding amount						397,300,000
Repayments of revolving credit					397,300,000
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
Line of credit facility, outstanding amount				$ 68,900,000	0
Repayment of term loan					195,000,000.0
Interest expense recorded				10,000,000.0	300,000	0
Deferred financing costs				$ 1,100,000
Average interest rate				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] \| Unsecured Credit Agreement September 8, 2017 [Member] \| Senior Term Loan Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum revolving credit									575,000,000.0
Credit agreement maturity date				Sep. 08, 2019
Interest expense recorded				$ 0	0	8,400,000
Parent Company [Member] \| Unsecured Credit Agreement September 8, 2017 [Member] \| Senior Term Loan Credit Agreement [Member] \| Newmark [Member]
Debt Instrument [Line Items]
Line of credit facility, outstanding amount				0	0	270,700,000
Line of credit facility, borrowing amount					270,700,000
Parent Company [Member] \| 8.125% Senior Notes Due 2042 [Member]
Debt Instrument [Line Items]
Interest expense recorded				$ 0	0
Principal amount of notes												112,500,000
Stated interest rate				8.125%
Debt maturity year				2042
Earliest call date				Jun. 26, 2017
Redemption price as percentage of principal amount				100.00%
Carrying amount of debt component												108,700,000
Debt issuance costs												$ 3,800,000
Parent Company [Member] \| 5.375% Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Interest expense recorded				$ 0	0
Principal amount of notes											$ 300,000,000.0
Stated interest rate				5.375%
Debt maturity year				2019
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
Parent Company [Member] \| 5.125% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Interest expense recorded				$ 16,200,000	16,200,000	16,200,000
Principal amount of notes										300,000,000.0
Stated interest rate				5.125%
Debt maturity year				2021
Redemption price as percentage of principal amount				101.00%
Carrying amount of debt component				$ 298,700,000						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
Parent Company [Member] \| 5.375% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Interest expense recorded				$ 25,600,000	11,000,000.0	0
Principal amount of notes		450,000,000.0
Stated interest rate				5.375%
Debt maturity year				2023
Redemption price as percentage of principal amount				101.00%
Carrying amount of debt component		444,200,000		$ 445,200,000
Debt issuance costs		$ 5,800,000
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 due 2023 will mature on July 24, 2023
Senior Notes maturity date				Jul. 24, 2023
Parent Company [Member] \| 3.750% Senior Notes Due October 1, 2024 [Member]
Debt Instrument [Line Items]
Interest expense recorded				$ 3,200,000	0	0
Principal amount of notes							300,000,000.0
Stated interest rate				3.75%
Carrying amount of debt component				$ 296,100,000			296,100,000
Debt issuance costs							$ 3,900,000
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
Parent Company [Member] \| Unsecured Credit Agreement [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum revolving credit			$ 150,000,000.0
Credit agreement maturity date			Sep. 08, 2017
Line of credit facility, outstanding amount				$ 0	$ 0
Interest expense recorded						$ 2,400,000
Maximum revolving credit terminated			$ 25,000,000.0
Debt instrument, description of reference rate				LIBOR
Line of credit facility, description				Borrowings under this facility bore 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.
Maximum revolving credit, option to increase			$ 200,000,000.0
Line of credit facility, terminated outstanding amount									$ 150,000,000.0
Parent Company [Member] \| Unsecured Credit Agreement [Member] \| Revolving Credit Facility [Member] \| Maximum [Member] \| LIBOR or Defined Base Rate [Member]
Debt Instrument [Line Items]
Debt instrument, basis point			2.50%
Parent Company [Member] \| Unsecured Credit Agreement [Member] \| Revolving Credit Facility [Member] \| Minimum [Member] \| LIBOR or Defined Base Rate [Member]
Debt Instrument [Line Items]
Debt instrument, basis point			0.50%