Related Party Transactions - Transfer of Employees to Newmark and Other Related Party Transactions (Details) shares in Millions		1 Months Ended		9 Months Ended
	May 01, 2018 USD ($) Employee	Feb. 28, 2019 USD ($)	Oct. 31, 2018 USD ($)	Sep. 30, 2020 USD ($) shares	Jul. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Related Party Transaction
Stockholders equity				$ 648,756,000		$ 599,664,000
Redeemable partnership interests				22,608,000		21,517,000	$ 21,517,000	$ 26,170,000
Payables to related parties				$ 5,185,000		38,090,000
Newmark Holdings, L.P.
Related Party Transaction
Limited partnership units exchange ratio (in percentage)				0.4545
CCRE
Related Party Transaction
Number of employees transferred | Employee	5
Proceeds from limited partnership units			$ 6,900,000
Stockholders equity				$ 6,900,000
Redeemable partnership interests				$ 2,200,000
Maximum revenue share to related party in first two years	$ 3,300,000
Payables to related parties					$ 3,300,000	$ 2,600,000
CCRE | Newmark Holdings, L.P.
Related Party Transaction
Proceeds from issuance of EPUs			6,700,000
Limited partnership unit cash distributions			$ 200,000
Issuance of additional limited partners units in exchange (in shares) | shares				2.2
Issuance of additional limited partners units in exchange for cash payment				$ 500,000
Issuance of additional limited partnership units with capital account				$ 2,200,000
Cantor Fitzgerald Limited Partnership And B G C Partners Incorporation | Government Sponsored Enterprise Loans
Related Party Transaction
Maximum amount per loan		$ 100,000,000.0
Limit on loans that have not yet been acquired or sold		250,000,000.0
Limit on loans outstanding		$ 250,000,000.0