Compensation - Additional Information (Detail) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2021 USD ($) shares	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($) shares	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($) shares	Dec. 31, 2020 USD ($) shares	Jun. 22, 2016 shares	Jun. 21, 2016 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate number of shares grant of future awards (in shares)	72,900,000			72,900,000
Number of share-equivalent limited partnership units exchangeable into shares (in shares)	1,300,000			1,300,000		3,500,000
Number of common stock settled per restricted stock unit (in shares)				1
Aggregate estimated fair value of limited partnership units and restricted stock units | $		$ 9,400		$ 8,900
Aggregate estimated fair value of deferred compensation awards | $		$ 23,600		$ 20,700
Shares, forfeited (in shares)	56,000		0	140,000	0
Number of shares released the restrictions (in shares)	600,000		100,000	1,100,000	500,000
Deferred cash compensation expense recognized | $				$ 321	$ 502
Deferred cash compensation expense not yet recognized | $	$ 200			$ 200
Deferred compensation expense, expected period of recognition				2 years 1 month 28 days
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to unvested | $	$ 38,000			$ 38,000
Unrecognized compensation expense related to unvested recognized period				2 years 5 months 8 days
Limited partnership units outstanding (in shares)	11,047,000			11,047,000		8,960,000
Restricted Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding (in shares)	2,600,000			2,600,000		3,700,000
Minimum | Restricted Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares saleable period				5 years
Maximum | Restricted Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares saleable period				10 years
LPUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock exchangeable basis				1
Limited partnership interests exchange ratio				0.9339
Unrecognized compensation expense related to unvested | $	$ 97,600			$ 97,600
Unrecognized compensation expense related to unvested recognized period				2 years 3 days
Limited partnership units outstanding (in shares)	122,559,000			122,559,000		137,652,000
LPUs | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares, restriction period				2 years
LPUs | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares, restriction period				5 years
Newmark
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares released the restrictions (in shares)	400,000		100,000	500,000	200,000
Number of outstanding restricted shares held (in shares)	1,300,000			1,300,000		1,700,000
Newmark | LPUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding (in shares)	11,123,000			11,123,000		13,202,000
Newmark | LPUs | REUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Notional value with post-termination pay-out amount | $	$ 800			$ 800		$ 800
Aggregate estimated fair value of limited partnership units | $	$ 300			$ 300		$ 300
BGC Holdings | LPUs | REUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of unvested limited partnership units with post-termination pay-out (in shares)	1,300,000			1,300,000		1,300,000
Notional value with post-termination pay-out amount | $	$ 12,400			$ 12,400		$ 12,700
Aggregate estimated fair value of limited partnership units | $	$ 7,500			$ 7,500		$ 7,500
Number of outstanding limited partnership units, unvested (in shares)	100,000			100,000		100,000
GFI Group Inc.
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred cash compensation expense recognized | $	$ 32		$ 100	$ 300	$ 500
Liability for deferred cash compensation awards | $	$ 700			$ 700		$ 1,500
Class A Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of share-equivalent limited partnership units exchangeable into shares (in shares)	400,000			400,000		500,000
Shares, forfeited (in shares)	56,423		0	140,188	0
Class A Common Stock | Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Value of shares withheld | $	$ 26		$ 69	$ 3,800	$ 1,800
Equity Plan | Class A Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized to be delivered pursuant to awards granted (in shares)							400,000,000	350,000,000