Common stock exchangeable basis.

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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 fair value of limited partnership units and restricted stock units.

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Limited partnership interests exchange ratio, subject to adjustment.

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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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Restricted Stock Units Vesting Description

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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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- Definition

The compensation expense recognized during the period pertaining to the 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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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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- Definition

The number of limited partner units outstanding.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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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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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.

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

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Number of shares authorized for issuance under share-based payment arrangement.

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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 ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jun. 22, 2016	Jun. 21, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate number of shares grant of future awards	128,200,000		128,200,000
Common stock exchangeable basis			one-for-one basis
Limited partnership interests Exchange Ratio			93.73%
Number of share-equivalent limited partnership units exchangeable into shares	2,200,000		2,200,000		3,100,000
Restricted stock unit, Conversion description			Each RSU is settled in one share of Class A common stock upon completion of the vesting period.
Aggregate estimated fair value of limited partnership units and restricted stock units			$ 8,700		$ 10,700
Shares, Forfeited	0	22,000	0	22,000
Number of shares released the restrictions	100,000	100,000	500,000	600,000
Deferred cash compensation expense recognized			$ 502	$ 1,193
Deferred cash compensation expense not yet recognized	$ 700		$ 700
Deferred compensation expense, expected period of recognition			1 year 6 months
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to unvested	$ 28,500		$ 28,500
Unrecognized compensation expense related to unvested recognized period			2 years 7 months 6 days
Limited partnership units outstanding	8,254,000		8,254,000		4,478,000
Restricted Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding	5,300,000		5,300,000		6,400,000
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 Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares saleable period			10 years
LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to unvested	$ 88,300		$ 88,300
Unrecognized compensation expense related to unvested recognized period			1 year 11 months 8 days
Limited partnership units outstanding	136,857,000		136,857,000		102,407,000
LPUs [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares, restriction period			2 years
LPUs [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]
Number of shares released the restrictions	55,000	42,000	200,000	300,000
Newmark [Member] \| LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding	13,852,000		13,852,000		14,607,000
Newmark [Member] \| LPUs [Member] \| REUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Notional value with post-termination pay-out amount	$ 900		$ 900		$ 1,000
Aggregate estimated fair value of limited partnership units	$ 300		$ 300		$ 200
BGC Holdings [Member] \| LPUs [Member] \| REUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of unvested limited partnership units with post-termination pay-out	1,300,000		1,300,000		1,200,000
Notional value with post-termination pay-out amount	$ 12,700		$ 12,700		$ 15,300
Aggregate estimated fair value of limited partnership units	$ 7,300		$ 7,300		$ 10,000
Number of outstanding limited partnership units, unvested	100,000		100,000		100,000
GFI Group Inc. [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred cash compensation expense recognized	$ 100	$ 600	$ 500	$ 1,200
Liability for deferred cash compensation awards	$ 3,700		$ 3,700		$ 3,500
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	600,000		600,000		800,000
Shares, Forfeited		22,000		22,000
Class A Common Stock [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Value of shares withheld	$ 69	$ 36	$ 1,800	$ 500
Class A Common Stock [Member] \| Newmark [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership interests Exchange Ratio			93.73%
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