Limited Partnership Interest in Newmark Holdings - Additional Information (Detail) shares in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018 Partnership shares	Dec. 31, 2018
Noncontrolling Interest [Abstract]
Payout period for post-termination awards				Four equal yearly installments
Percentage to preferred units				0.6875%
CF Group Management, Inc. [Member]
Noncontrolling Interest [Abstract]
Number of shares, right to exchange from Class A to Class A or Class B common stock				34.6
Cantor Rights to Purchase Exchangeable Units [Member] | Maximum [Member]
Noncontrolling Interest [Abstract]
Number of shares, right to exchange from Class A to Class A or Class B common stock				34.6
Scenario, Forecast [Member]
Noncontrolling Interest [Abstract]
Percentage to preferred units	0.6875%	0.6875%	0.6875%		2.75%
BGC Partners, Inc. [Member]
Noncontrolling Interest [Abstract]
Number of operating partnerships | Partnership				2
BGC Holdings Limited Partnership [Member]
Noncontrolling Interest [Abstract]
Limited partnership interest initial contribution ratio				45.45%
Limited partnership interest contribution ratio				47.02%