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Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 13, 2018	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	154,400,000
Equity-based compensation expense	$ 1,100
Number of share-equivalent limited partnership units exchangeable into shares	2,800,000			3,900,000
Number of unvested limited partnership units with post-termination pay-out			300,000
Number of outstanding limited partnership units, unvested			100,000
Aggregate estimated fair value of limited partnership units			$ 4,800
Income allocated to limited partnership units	$ 4,500	$ 3,800
Restricted stock unit, Conversion description	Each RSU is settled in one share of Class A common stock upon completion of the vesting period.
Shares, Forfeited	0	0
Number of shares released the restrictions	500,000	500,000
Cash received per share upon conversion	$ 6.10
Deferred cash compensation expense recognized	$ (7)	$ 1,311
Deferred cash compensation expense not yet recognized	$ 2,000
Deferred compensation expense, expected period of recognition	1 year 9 months 14 days
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense	$ 700	$ 1,200
Awards granted	138,784	49,000
Aggregate estimated grant date fair values	$ 800	$ 600
Value of shares withheld	400	1,300
Aggregate estimated grant date fair value of outstanding RSUs	7,800		$ 10,100
Unrecognized compensation expense related to unvested RSUs	7,300
Share based compensation related acquisition	$ 300	$ 0
Limited partnership units outstanding	766,930		928,713
Restricted Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding	6,900,000
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]
Number of limited partnership units granted exchangeability		1,500,000
Equity-based compensation expense	$ 5,000	$ 34,500
Aggregate estimated fair value of limited partnership units	3,600		$ 3,600
Compensation expense before income tax related to limited partnership units	$ (500)	$ 200
Awards granted	9,290,951
Limited partnership units outstanding	80,943,527		79,729,188
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
BGC Holdings [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Redemptions of units	200,000
Notional value with post-termination pay-out amount	$ 10,000		$ 10,000
Number of unvested limited partnership units with post-termination pay-out	300,000		300,000
Number of outstanding limited partnership units, unvested	59,000		68,000
Aggregate estimated fair value of limited partnership units	$ 5,100
BGC Holdings [Member] \| Limited Partnership Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of limited partnership units granted exchangeability	300,000	3,200,000
Newmark [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Redemptions of units	100,000
Notional value with post-termination pay-out amount	$ 1,000
Number of unvested limited partnership units with post-termination pay-out	129,000		200,000
Number of outstanding limited partnership units, unvested	85,000		31,000
Aggregate estimated fair value of limited partnership units	$ 100
Newmark [Member] \| Limited Partnership Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of limited partnership units granted exchangeability	100,000	1,400,000
Awards granted	508,522
Limited partnership units outstanding	15,488,589		22,113,105
GFI Group Inc. [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred cash compensation expense recognized	$ 1,300	$ 1,300
Liability for deferred cash compensation awards	$ 5,500
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