Limited Partnership Interests - Additional Information (Detail) shares in Millions, $ in Millions	3 Months Ended				9 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019 USD ($)	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2019 USD ($) shares	Dec. 31, 2019	Nov. 30, 2018	Sep. 26, 2018 USD ($)	Jun. 18, 2018 USD ($)
Noncontrolling Interest [Abstract]
Payout period for post-termination awards					four equal yearly installments
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction		93.62%			93.62%
Percentage to preferred units		0.6875%	0.6875%	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 | shares					23.0
Cantor Rights to Purchase Exchangeable Units [Member]
Noncontrolling Interest [Abstract]
Number of shares, right to exchange from Class A to Class A or Class B common stock | shares					23.0
BGC Holdings, L.P. [Member]
Noncontrolling Interest [Abstract]
Limited partnership interest initial contribution ratio					45.45%
Exchange Ratio Number of Common Shares Received Under Spin Off Transaction							46.3895%
Percentage to preferred units		0.6875%	0.6875%	0.6875%
Scenario, Forecast [Member]
Noncontrolling Interest [Abstract]
Percentage to preferred units	0.6875%					2.75%
Scenario, Forecast [Member] | BGC Holdings, L.P. [Member]
Noncontrolling Interest [Abstract]
Percentage to preferred units	0.6875%					2.75%
Class A Common Stock [Member]
Noncontrolling Interest [Abstract]
Limited partnership units exchange ratio					0.9362
Newmark OpCo [Member] | RBC [Member]
Noncontrolling Interest [Abstract]
Exchangeable preferred limited partnership units issued | $		$ 325.0			$ 325.0			$ 150.0	$ 175.0
Exchangeable preferred limited partnership units | $					$ 325.0