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Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 13, 2017	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		184,700,000
Number of share-equivalent limited partnership units exchangeable into shares	20,000,000	22,900,000		20,000,000
Number of standalone limited partnership units exchangeable shares		900,000
Notional value with post-termination pay-out amount	$ 242,100	$ 148,700		$ 242,100
Aggregate estimated fair value of limited partnership units	$ 45,700	$ 35,100		$ 45,700
Number of unvested limited partnership units with post-termination pay-out	24,100,000	14,500,000		24,100,000
Number of outstanding limited partnership units, unvested	13,300,000	6,700,000		13,300,000
Income allocated to limited partnership units		$ 9,000	$ 9,400
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		0	0
Number of shares released the restrictions		500,000	1,000,000
Cash received per share upon conversion		$ 6.10
Deferred cash compensation expense recognized		$ 2,379	$ 3,777
Liability for deferred cash compensation awards		8,200
Deferred cash compensation expense not yet recognized		$ 5,300
Deferred compensation expense, expected period of recognition		1 year 10 months 20 days
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding	1,409,747	1,019,100		1,409,747
Number of units, granted		51,976	100,000
Number of units, forfeited		(29,572)
Equity-based compensation expense		$ 1,500	$ 1,100
Aggregate estimated grant date fair values		600	1,000
Value of shares withheld		1,700	$ 2,000
Aggregate estimated grant date fair value of outstanding RSUs	$ 12,300	9,200		$ 12,300
Unrecognized compensation expense related to unvested RSUs		$ 9,000
Weighted average remaining contractual term		1 year 8 months 8 days		1 year 8 months 4 days
Restricted Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding		8,700,000
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	129,306,880		123,631,195
Number of units, granted		11,343,446
Number of units, redeemed/exchanged		(5,629,072)
Number of units, forfeited		(38,689)
Number of limited partnership units granted exchangeability		4,700,000	5,400,000
Equity-based compensation expense		$ 56,200	$ 53,800
Aggregate estimated fair value of limited partnership units	$ 20,600	16,700		$ 20,600
Compensation expense (benefit) before income tax related to limited partnership units		$ (8,700)	6,400
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]
Number of share-equivalent limited partnership units exchangeable into shares	9,100,000	10,800,000		9,100,000
Notional value with post-termination pay-out amount		$ 21,600
Number of unvested limited partnership units with post-termination pay-out	11,000,000	6,600,000		11,000,000
Number of outstanding limited partnership units, unvested	6,000,000	3,000,000		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		40,000
Aggregate estimated grant date fair values		$ 500
Aggregate estimated grant date fair value of outstanding RSUs		$ 500
Weighted average remaining contractual term		2 years 5 months 4 days
Newmark [Member] \| Limited Partnership Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding	56,200,000	59,800,000		56,200,000
Number of units, increase in period		3,600,000
Number of units, granted		6,100,000
Number of units, redeemed/exchanged		(2,500,000)
Number of units, forfeited		(17,600)
Number of limited partnership units granted exchangeability		2,100,000
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		$ 2,100
Number of unvested limited partnership units with post-termination pay-out		1,500,000
Number of outstanding limited partnership units, unvested		1,100,000
GFI Group Inc. [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred cash compensation expense recognized		$ 1,300	3,800
BPF [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred cash compensation expense recognized		1,100	$ 200
Liability for deferred cash compensation awards	$ 400	$ 600		$ 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	22,900,000		20,000,000
Shares, Forfeited				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]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized to be delivered pursuant to awards granted					50,000,000
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] \| 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 Long Term Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate number of shares grant of future awards		50,000,000