Compensation - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($) shares	Jun. 30, 2020 USD ($) shares	Jun. 30, 2021 USD ($) shares	Jun. 30, 2020 USD ($) 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)	86,400,000		86,400,000
Number of share-equivalent limited partnership units exchangeable into shares (in shares)	2,900,000		2,900,000		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 | $			$ 9,300		$ 9,400
Aggregate estimated fair value of deferred compensation awards | $			$ 19,800		$ 23,600
Shares, forfeited (in shares)	84,000	0	84,000	0
Number of shares released the restrictions (in shares)	200,000	200,000	400,000	300,000
Deferred cash compensation expense recognized | $			$ 289	$ 404
Deferred cash compensation expense not yet recognized | $	$ 200		$ 200
Deferred compensation expense, expected period of recognition			2 years 2 months 23 days
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to unvested | $	$ 40,800		$ 40,800
Unrecognized compensation expense related to unvested recognized period			2 years 7 months 24 days
Limited partnership units outstanding (in shares)	10,801,000		10,801,000		8,960,000
Restricted Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding (in shares)	3,300,000		3,300,000		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			0.9403
Unrecognized compensation expense related to unvested | $	$ 81,500		$ 81,500
Unrecognized compensation expense related to unvested recognized period			1 year 10 months 20 days
Limited partnership units outstanding (in shares)	118,690,000		118,690,000		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)	0	100,000	100,000	100,000
Number of outstanding restricted shares held (in shares)	1,600,000		1,600,000		1,700,000
Newmark | LPUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units outstanding (in shares)	11,519,000		11,519,000		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 | $	$ 12,700		$ 12,700		$ 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 | $	$ 200	$ 200	$ 300	$ 400
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)	83,765	0	83,765	0
Class A Common Stock | Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Value of shares withheld | $	$ 600	$ 40	$ 3,800	$ 1,700
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