Compensation - Activity Associated with Restricted Stock Units (Details) - Restricted Stock Units - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended	12 Months Ended
	Jan. 01, 2018	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Newmark Holdings, L.P.
Restricted Stock Units
Beginning balance (in shares)		12,133,105	11,263,862
Number of units, Issued (in shares)		522,958	4,192,685
Number of Settled units (delivered shares) (in shares)		(1,517,298)	(2,708,902)
Number of units, Forfeited units (in shares)		(249,743)	(614,540)
Ending balance (in shares)		10,889,022	12,133,105	11,263,862
Weighted- Average Grant Date Fair Value Per Share
Balance at the beginning of period (in dollars per share)		$ 8.59	$ 9.39
Granted (in dollars per share)		7.27	6.82
Settled units (delivered shares) (in dollars per share)		9.01	9.03
Forfeited units (in dollars per share)		8.87	9.14
Balance at the end of period (in dollars per share)		$ 8.46	$ 8.59	$ 9.39
Fair Value Amount
Balance at beginning of period		$ 104,251	$ 105,735
Granted		3,802	28,594
Settled units (delivered shares)		(13,673)	(24,461)
Forfeited units		(2,214)	(5,617)
Balance at end of period		$ 92,166	$ 104,251	$ 105,735
Weighted- Average Remaining Contractual Term (Years)		4 years 4 months 2 days	4 years 3 days	4 years 9 months
Newmark Holdings, L.P. | Minimum
Fair Value Amount
Award vesting period	2 years
Newmark Holdings, L.P. | Maximum
Fair Value Amount
Award vesting period	9 years
BGC Holdings, L.P.
Restricted Stock Units
Beginning balance (in shares)		4,883	6,928
Number of units, Issued (in shares)		0	0
Number of Settled units (delivered shares) (in shares)		(1,243)	(2,045)
Number of units, Forfeited units (in shares)		0	0
Ending balance (in shares)		3,640	4,883	6,928
Weighted- Average Grant Date Fair Value Per Share
Balance at the beginning of period (in dollars per share)		$ 4.22	$ 4.17
Granted (in dollars per share)		0	0
Settled units (delivered shares) (in dollars per share)		4.28	4.05
Forfeited units (in dollars per share)		0	0
Balance at the end of period (in dollars per share)		$ 4.20	$ 4.22	$ 4.17
Fair Value Amount
Balance at beginning of period		$ 21	$ 29
Granted		0	0
Settled units (delivered shares)		(5)	(8)
Forfeited units		0	0
Balance at end of period		$ 16	$ 21	$ 29
Weighted- Average Remaining Contractual Term (Years)		8 months 4 days	10 months 13 days	1 year 7 months 13 days
BGC Holdings, L.P. | Minimum
Fair Value Amount
Award vesting period		2 years
BGC Holdings, L.P. | Maximum
Fair Value Amount
Award vesting period		3 years