Aggregate exchangeable limited partnership units outstanding.

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Cash per unit received upon conversion of deferred cash award.

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Deferred compensation arrangement with individual compensation expense not yet recognized.

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Deferred compensation arrangement with individual compensation expense not yet recognized period for recognition.

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Estimated fair value of limited partnership units held by executives and non-executive employees, awarded in lieu of cash compensation for salaries, commissions and/or discretionary or guaranteed bonuses.

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Estimated grant date fair value of outstanding RSU.

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Number of limited partnership units granted exchangeability into the company's class A common stock during the period.

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Notional value of limited partnership units with post termination pay out amount.

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Number of limited partnership units exchangeable into shares.

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Number of standalone limited partnership units exchangeable shares.

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Restricted Stock Units Vesting Description

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Share based compensation arrangement by share based payment award equity instruments other than options outstanding grant date fair value.

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Number of exchangeable limited partnership units redeemed or exchanged into the Company's class A common stock during the period.

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Share based compensation arrangement by share based payment award non option equity instruments increase in period.

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Compensation expense (benefit) before income tax related to limited partnership units that were not redeemed.

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Period over which the sale or transferability of an the equity-based payment award is restricted, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Restricted shares that were previously issued and then had their restriction released during the period.

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Value of shares withheld for tax, related to share-based compensation.

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Number of shares issued pursuant to the terms of a deferred compensation arrangement.

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Aggregate carrying value as of the balance sheet date of the liabilities for deferred compensation arrangements. Represents currently earned compensation under cash arrangements (such as a profit-sharing plan, rabbi trust, and employee contract--excluding equity-based arrangements) that is not actually paid until a later date.

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Amount of equity impact of cash distribution declared to unit-holder of limited partnership (LP).

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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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The number of limited partner units outstanding.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of shares under non-option equity instrument agreements that were cancelled as a result of occurrence of a terminating event.

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Net number of non-option equity instruments granted to participants.

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Number of equity instruments other than options outstanding, including both vested and non-vested instruments.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Number of shares related to Restricted Stock Award forfeited during the period.

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Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 13, 2017	Jun. 22, 2016	Jun. 21, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of share-equivalent limited partnership units exchangeable into shares	20,000,000	24,000,000		24,000,000		20,000,000
Number of standalone limited partnership units exchangeable shares		1,000,000		1,000,000
Notional value with post-termination pay-out amount	$ 242,100	$ 118,000		$ 118,000		$ 242,100
Aggregate estimated fair value of limited partnership units	$ 45,700	$ 32,600		$ 32,600		$ 45,700
Number of unvested limited partnership units with post-termination pay-out	24,100,000	11,100,000		11,100,000		24,100,000
Number of outstanding limited partnership units, unvested	13,300,000	4,300,000		4,300,000		13,300,000
Income allocated to limited partnership units		$ 44,400	$ 28,600	$ 63,100	$ 50,000
Restricted stock unit, Conversion description				Each BGC RSU is settled in one share of BGC Class A common stock upon completion of the vesting period.
Shares, Forfeited				194,000	107,000
Number of shares released the restrictions				1,200,000	2,100,000
Cash received per share upon conversion				$ 6.10
Deferred cash compensation expense recognized				$ 5,553	$ 7,265
Liability for deferred cash compensation awards		4,400		4,400
Deferred cash compensation expense not yet recognized		$ 3,100		$ 3,100
Deferred compensation expense, expected period of recognition				1 year 6 months 7 days
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding	1,409,747	1,123,463		1,123,463		1,409,747
Number of units, granted				460,402	800,000
Number of units, forfeited				(120,110)
Equity-based compensation expense		$ 1,200	$ 1,600	$ 4,900	$ 4,300
Aggregate estimated grant date fair values				5,700	7,800
Value of shares withheld				2,200	$ 2,300
Aggregate estimated grant date fair value of outstanding RSUs	$ 12,300	11,400		11,400		$ 12,300
Unrecognized compensation expense related to unvested RSUs		$ 12,600		$ 12,600
Weighted average remaining contractual term				1 year 8 months 15 days		1 year 8 months 4 days
Weighted-average grant date fair value per share	$ 8.76	$ 10.16		$ 10.16		$ 8.76
Restricted Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding		7,700,000		7,700,000
Minimum [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares, restriction period				2 years
Minimum [Member] \| Restricted Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares saleable period				5 years
Maximum [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares, restriction period				4 years
Maximum [Member] \| Restricted Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares saleable period				10 years
Limited Partnership Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding	123,631,195	138,531,539		138,531,539		123,631,195
Number of units, granted				28,836,257
Number of units, redeemed/exchanged				(13,897,224)
Number of units, forfeited				(38,689)
Number of limited partnership units granted exchangeability		1,900,000	1,800,000	14,700,000	10,700,000
Equity-based compensation expense		$ 23,500	$ 19,800	$ 176,600	$ 111,900
Aggregate estimated fair value of limited partnership units	$ 20,600	16,100		16,100		$ 20,600
Compensation expense (benefit) before income tax related to limited partnership units		$ 1,100	6,600	$ (6,200)	19,000
Limited Partnership Units [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares, restriction period				3 years
Limited Partnership Units [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares, restriction period				5 years
Newmark [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate exchangeable limited partnership units		11,200,000		11,200,000
Number of share-equivalent limited partnership units exchangeable into shares	9,100,000	11,400,000		11,400,000		9,100,000
Notional value with post-termination pay-out amount		$ 62,300		$ 62,300
Number of unvested limited partnership units with post-termination pay-out	11,000,000	5,200,000		5,200,000		11,000,000
Number of outstanding limited partnership units, unvested	6,000,000	2,000,000		2,000,000		6,000,000
Newmark [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of units, granted		53,000		265,000
Aggregate estimated grant date fair values		$ 600		$ 3,800
Aggregate estimated grant date fair value of outstanding RSUs		$ 3,700		$ 3,700
Weighted average remaining contractual term				2 years 5 months 15 days
Weighted-average grant date fair value per share		$ 14.19		$ 14.19
Newmark [Member] \| Limited Partnership Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding	56,200,000	64,500,000		64,500,000		56,200,000
Number of units, increase in period				8,300,000
Number of units, granted				15,500,000
Number of units, redeemed/exchanged				(7,200,000)
Number of units, forfeited				(17,600)
Number of limited partnership units granted exchangeability		1,000,000		6,100,000
Newmark [Member] \| Standalone Limited Partnership Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate estimated fair value of limited partnership units		$ 6,000		$ 6,000
Number of unvested limited partnership units with post-termination pay-out		4,200,000		4,200,000
Number of outstanding limited partnership units, unvested		2,700,000		2,700,000
GFI Group Inc. [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred cash compensation expense recognized		$ 1,300	1,000	$ 3,800	7,200
BPF [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred cash compensation expense recognized		400	$ (300)	1,700	$ 100
Liability for deferred cash compensation awards	$ 400	$ 1,200		$ 1,200		$ 400
Class A Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of share-equivalent limited partnership units exchangeable into shares	20,000,000	24,000,000		24,000,000		20,000,000
Shares, Forfeited		49,189	71,251	193,819	106,849	183,246
Class A Common Stock [Member] \| Chief Executive Officer and Chief Financial Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred stock issued				1,000,000
Class A Common Stock [Member] \| Newmark [Member] \| Chief Executive Officer and Chief Financial Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred cash compensation expense recognized	$ 40,300
Deferred stock issued				300,000
Class A Common Stock [Member] \| Newmark [Member] \| Minimum [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares, restriction period				2 years
Class A Common Stock [Member] \| Newmark [Member] \| Maximum [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares, restriction period				3 years
Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining capacity of shares under equity plan		173,100,000		173,100,000
Equity Plan [Member] \| Class A Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized to be delivered pursuant to awards granted								400,000,000	350,000,000
Newmark Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining capacity of shares under equity plan		399,600,000		399,600,000
Newmark Equity Plan [Member] \| Class A Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized to be delivered pursuant to awards granted							400,000,000
Newmark Equity Plan [Member] \| Class A Common Stock [Member] \| Newmark [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized to be delivered pursuant to awards granted							50,000,000