Limited Partnership Interests - Additional Information (Detail) $ in Millions			3 Months Ended				9 Months Ended	12 Months Ended
	Jul. 01, 2018	Jun. 05, 2015 shares	Dec. 31, 2018	Sep. 30, 2018 USD ($)	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018 USD ($) shares	Dec. 31, 2018	Dec. 31, 2017 shares	Sep. 25, 2018 USD ($)	Jun. 18, 2018 USD ($)
Noncontrolling Interest [Abstract]
Number of operating partnerships							2
Limited partnership interest contribution ratio							45.45%
Payout period for post-termination awards							four equal yearly installments
Percentage to preferred units				0.6875%	0.6875%	0.6875%
Distribution ratio	46.40%				46.47%	45.45%
CF Group Management, Inc. [Member]
Noncontrolling Interest [Abstract]
Number of shares, right to exchange from Class A to Class B common stock		34,649,693
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		34,649,693
Scenario Forecast [Member]
Noncontrolling Interest [Abstract]
Percentage to preferred units			0.6875%					2.75%
Newmark Partners L.P [Member] | RBC [Member]
Noncontrolling Interest [Abstract]
Exchangeable preferred limited partnership units issued | $				$ 325			$ 325			$ 150	$ 175