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, 2019	Sep. 30, 2018	Dec. 31, 2019
Noncontrolling Interest [Abstract]
Number of operating partnerships								2
Limited partnership interest contribution ratio								45.45%
Limited partnership interests Exchange Ratio								93.62%
Payout period for post-termination awards								four equal yearly installments
Percentage to preferred units					0.6875%	0.6875%	0.6875%
Distribution ratio	46.3895%								45.45%
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%						2.75%