Compensation - Activity Associated with Restricted Stock Units (Detail) - Restricted Stock Units (RSUs) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended	12 Months Ended
	Dec. 31, 2018	Mar. 31, 2020	Dec. 31, 2019
BGC Holdings, L.P.
Restricted Stock Units
Number of Units, Balance outstanding at beginning of period (in shares)		46,807	168,675
Number of Units, Granted (in shares)		0	0
Number of Settled units (delivered shares) (in shares)		(35,825)	(107,820)
Number of Units, Forfeited units (in shares)		(857)	(14,048)
Number of Units, Balance outstanding at end of period (in shares)	168,675	10,125	46,807
Weighted- Average Grant Date Fair Value Per Share
Balance at the beginning of period (in dollars per share)		$ 9.97	$ 9.77
Granted (in dollars per share)		0	0
Settled units (delivered shares) (in dollars per share)		10.32	9.38
Forfeited units (in dollars per share)		10.54	10.02
Balance at the end of period (in dollars per share)	$ 9.77	$ 8.71	$ 9.97
Fair Value Amount
Balance at beginning of period		$ 467	$ 1,619
Granted		0	0
Settled units (delivered shares)		(370)	(1,011)
Forfeited units		(9)	(141)
Balance at end of period	$ 1,619	$ 88	$ 467
Weighted- Average Remaining Contractual Term	29 days	5 days	3 months
BGC Holdings, L.P. | Minimum
Fair Value Amount
Award vesting period		2 years
BGC Holdings, L.P. | Maximum
Fair Value Amount
Award vesting period		4 years
Newmark Holdings, L.P.
Restricted Stock Units
Number of Units, Balance outstanding at beginning of period (in shares)		4,683,571	219,887
Number of Units, Granted (in shares)		1,686,149	4,766,611
Number of Settled units (delivered shares) (in shares)		(425,271)	(109,007)
Number of Units, Forfeited units (in shares)		(17,669)	(193,920)
Number of Units, Balance outstanding at end of period (in shares)	219,887	5,926,780	4,683,571
Weighted- Average Grant Date Fair Value Per Share
Balance at the beginning of period (in dollars per share)		$ 7.55	$ 13.52
Granted (in dollars per share)		9.58	7.42
Settled units (delivered shares) (in dollars per share)		8.50	11.70
Forfeited units (in dollars per share)		8.54	8.67
Balance at the end of period (in dollars per share)	$ 13.52	$ 8.06	$ 7.55
Fair Value Amount
Balance at beginning of period		$ 35,361	$ 2,973
Granted		16,153	35,344
Settled units (delivered shares)		(3,616)	(1,275)
Forfeited units		(151)	(1,681)
Balance at end of period	$ 2,973	$ 47,747	$ 35,361
Weighted- Average Remaining Contractual Term	2 years 3 months 10 days	6 years 29 days	5 years 8 months 8 days
Newmark Holdings, L.P. | Minimum
Fair Value Amount
Award vesting period		2 years
Newmark Holdings, L.P. | Maximum
Fair Value Amount
Award vesting period		8 years