Limited Partnership Interests in BGC Holdings and Newmark Holdings - Additional Information (Detail) - shares			3 Months Ended				6 Months Ended	9 Months Ended	12 Months Ended
	Nov. 23, 2018	Jun. 05, 2015	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2020	Sep. 30, 2018	Dec. 31, 2020
Noncontrolling Interest [Abstract]
Number of operating partnerships							2
Limited partnership interest contribution ratio							45.45%
Limited partnership interests Exchange Ratio							93.66%
Payout period for post-termination awards							four equal yearly installments
Percentage to preferred units					0.6875%	0.6875%
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
Scenario Forecast [Member]
Noncontrolling Interest [Abstract]
Percentage to preferred units			0.6875%	0.6875%					2.75%