Consolidated Statements of Changes in Equity (Parenthetical) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Grant of exchangeability, redemption and issuance of limited partnership interests and issuance of common stock (in shares)	3,867,127	2,052,183
Class A Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares in the Separation, shares			115,593,787
Issuance of Class A common stock for Newmark RSUs (in shares)			23,000,000
Equity-based compensation and related issuance, shares			600,000
Treasury stock repurchases (in shares)	4,518,002	50,000
Class B Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares in the Separation, shares			15,840,049
Noncontrolling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
BGC's purchase of exchangeable limited partnership units in Newmark Holdings, shares		16,606,726
Exchangeable Limited Partnership Units | Newmark Holdings, L.P.
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Number of units converted to common stock (in shares)		14,831,234
Exchangeable Limited Partnership Units | Newmark OpCo
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Number of units converted to common stock (in shares)		6,903,876
Exchangeable Limited Partnership Units | Class A Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares in exchange of exchangeable limited partnership units (in shares)		16,292,623
Exchangeable Limited Partnership Units | Class B Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares in exchange of exchangeable limited partnership units (in shares)		5,445,488