Limited Partnership Interests in BGC Holdings and Newmark Holdings - Additional Information (Detail) - shares				3 Months Ended				9 Months Ended	12 Months Ended
	Nov. 30, 2018	Nov. 23, 2018	Jun. 05, 2015	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018
Noncontrolling Interest [Abstract]
Number of operating partnerships									2
Limited partnership interest contribution ratio									45.45%
Limited partnership interests Exchange ratio,									94.00%
Payout period for post-termination awards									four equal yearly installments
Percentage to preferred units				0.6875%	0.6875%	0.6875%	0.6875%		2.75%
Number of shares, right to exchange from Class A to Class B common stock										11,036,273
Distribution ratio	46.3895%							45.45%
Common stock exchangeable basis									one-for-one basis
Newmark [Member]
Noncontrolling Interest [Abstract]
Common stock exchangeable basis	one-for-one
CF Group Management, Inc. [Member]
Noncontrolling Interest [Abstract]
Number of shares, right to exchange from Class A to Class B common stock		712,907	34,649,693						23,600,000
Cantor Rights to Purchase Exchangeable Units [Member]
Noncontrolling Interest [Abstract]
Number of shares, right to exchange from Class A to Class B common stock		10,323,366
Common stock exchangeable basis									one-for-one
Cantor Rights to Purchase Exchangeable Units [Member] | Maximum [Member]
Noncontrolling Interest [Abstract]
Number of shares, right to exchange from Class A to Class B common stock			34,649,693						23,600,000