Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended					11 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018 USD ($)	Nov. 30, 2018 Segment	Dec. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | Segment						2	1
Maturity days for highly liquid investments							The Company considers all highly liquid investments with maturities of 90 days or less at the date of acquisition that are not segregated under regulatory requirements, other than those used for trading purposes, to be cash equivalents.
Allowance for doubtful accounts on accrued commissions receivable	$ 10,500,000				$ 8,700,000		$ 10,500,000	$ 8,700,000
Ownership interest							The Company’s policy is to consolidate all entities of which it owns more than 50% unless it does not have control over the entity.
Impairment of goodwill	$ 0				$ 0		$ 0	0	$ 0
Impairment of goodwill or indefinite-lived intangible assets							$ 0	$ 0	$ 0
US federal corporate tax rate							21.00%		35.00%
Restricted stock saleable period Description							shares of restricted stock are generally saleable by partners in five to ten years.
Payout period for post-termination awards							four equal yearly installments
Limited partnership units exchangeability into Class A common stock							Following the Spin-Off, certain limited partnership units in BGC Holdings are granted exchangeability into BGC Class A common stock on a one-for-one basis (subject to adjustment), and certain limited partnership units in Newmark Holdings are granted exchangeability into Newmark Class A common stock based on the exchange ratio at the time.
Percentage to preferred units	0.6875%	0.6875%	0.6875%	0.6875%			2.75%
Software, Including Software Development Costs [Member]
Significant Accounting Policies [Line Items]
Software amortization period							3 years
Maximum [Member]
Significant Accounting Policies [Line Items]
Restricted stock saleable period							10 years
Maximum [Member] | Computer and Communications Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of computer software							5 years
Minimum [Member]
Significant Accounting Policies [Line Items]
Restricted stock saleable period							5 years
Minimum [Member] | Computer and Communications Equipment [Member]
Significant Accounting Policies [Line Items]
Useful life of computer software							3 years