Limited Partnership Interests in Newmark Holdings and BGC Holdings (Details) $ in Millions	3 Months Ended					12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2019 USD ($) shares	Sep. 26, 2018 USD ($) tranche	Jun. 18, 2018 USD ($)	Dec. 13, 2017
Related Party Transaction [Line Items]
Payout period for post-termination awards						4 years
Percentage to preferred units	0.6875%	0.6875%	0.6875%	0.6875%	0.6875%	2.75%
Non Distribution Earning Units
Related Party Transaction [Line Items]
Award vesting period						4 years
Class A Common Stock
Related Party Transaction [Line Items]
Limited partnership units exchange ratio	0.9400					0.9400			1
BGC Holdings, L.P.
Related Party Transaction [Line Items]
Limited partnership interest initial contribution ratio						45.45%
BGC Holdings, L.P. | Minimum
Related Party Transaction [Line Items]
Exchange ratio (per share)	46.3895%					46.3895%
Cantor Rights to Purchase Exchangeable Units
Related Party Transaction [Line Items]
Number of shares, right to exchange from Class A to Class A or Class B common stock (in shares) | shares						21,300,000
Newmark OpCo | RBC
Related Party Transaction [Line Items]
Exchangeable preferred limited partnership units issued (in shares) | $	$ 325.0					$ 325.0	$ 150.0	$ 175.0
Exchangeable preferred limited partnership units, number of tranches | tranche							4