Compensation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 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	144,600,000		144,600,000
Equity-based compensation expense	$ 21,300		$ 48,400
Number of share-equivalent limited partnership units exchangeable into shares	2,700,000		2,700,000		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	$ 10,300	$ 20,700	$ 19,600	$ 32,600
Restricted stock unit, Conversion description			Each RSU is settled in one share of BGC Class A common stock upon completion of the vesting period.
Shares, Forfeited		49,189	22,000	193,819
Number of shares released the restrictions			1,200,000	1,200,000
Deferred cash compensation expense recognized			$ 1,193	$ 3,766
Deferred cash compensation expense not yet recognized	4,800		$ 4,800
Deferred compensation expense, expected period of recognition			2 years 2 months 12 days
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense	$ 1,600	$ 1,000	$ 5,300	$ 3,600
Awards granted	300,000	100,000	3,705,771	500,000
Aggregate estimated grant date fair values	$ 1,200	$ 1,000	$ 17,100	$ 5,700
Value of shares withheld	36	$ 100	500	$ 1,500
Aggregate estimated grant date fair value of outstanding RSUs	21,800		21,800		$ 10,100
Unrecognized compensation expense related to unvested RSUs	$ 18,200		$ 18,200
Limited partnership units outstanding	4,045,395		4,045,395		928,713
Restricted Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding	6,400,000		6,400,000
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
LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of limited partnership units granted exchangeability		900,000		7,700,000
Equity-based compensation expense	$ 3,000	$ 11,300	$ 4,900	$ 82,200
Redemptions of LPUs			12,913,487
Aggregate estimated fair value of limited partnership units	3,400		$ 3,400		$ 3,600
Compensation expense before income tax related to limited partnership units	(100)	1,200	(600)	600
Compensation expense before income tax related to limited partnership units, no quarterly allocation of net income	$ 4,300	$ 700	$ 18,500	$ 1,000
Awards granted			37,119,394
Limited partnership units outstanding	99,870,993		99,870,993		79,729,188
LPUs [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares, restriction period			3 years
Restricted shares, restriction period, no quarterly allocation of net income			2 years
LPUs [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares, restriction period			5 years
Restricted shares, restriction period, no quarterly allocation of net income			5 years
BGC Holdings [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Redemptions of LPUs	4,400,000	0	9,900,000	0
Notional value with post-termination pay-out amount	$ 10,000		$ 10,000		$ 10,000
Number of unvested limited partnership units with post-termination pay-out	300,000		300,000		300,000
Number of outstanding limited partnership units, unvested	57,000		57,000		68,000
Aggregate estimated fair value of limited partnership units	$ 5,000		$ 5,000
BGC Holdings [Member] | Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Notional value with post-termination pay-out amount	$ 5,000		$ 5,000
Number of outstanding limited partnership units, unvested	800,000		800,000
Aggregate estimated fair value of limited partnership units	$ 4,800		$ 4,800
BGC Holdings [Member] | LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of limited partnership units granted exchangeability	300,000	1,000,000.0	600,000	7,200,000
Newmark [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Redemptions of LPUs	100,000	0	300,000	0
Notional value with post-termination pay-out amount	$ 1,000		$ 1,000
Number of unvested limited partnership units with post-termination pay-out	127,000		127,000		200,000
Number of outstanding limited partnership units, unvested	69,000		69,000		31,000
Aggregate estimated fair value of limited partnership units	$ 200		$ 200
Newmark [Member] | LPUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of limited partnership units granted exchangeability	100,000	500,000	200,000	3,400,000
Redemptions of LPUs			592,362
Awards granted			611,637
Limited partnership units outstanding	15,013,388		15,013,388		22,113,105
GFI Group Inc. [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred cash compensation expense recognized	$ 600	$ 1,500	$ 1,200	$ 3,800
Liability for deferred cash compensation awards	$ 6,000		$ 6,000
Class A Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Forfeited	22,046	49,189	22,046	193,819
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