Compensation - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($) shares	Sep. 30, 2022 USD ($) shares	Sep. 30, 2023 USD ($) shares	Sep. 30, 2022 USD ($) shares	Dec. 31, 2022 shares	Dec. 31, 2021 shares	Dec. 13, 2017 shares
Share-based Compensation Arrangement by Share-based Payment Award
Shares available for future awards (in shares)	309,500,000		309,500,000
Limited partnership units exchange ratio	0.4545		0.4545
Issuance of common stock and exchangeability expenses | $	$ 19,121,000	$ 33,330,000	$ 53,601,000	$ 71,211,000
Unrecognized compensation expense related to unvested RSUs | $	101,900,000		101,900,000
Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Issuance of common stock and exchangeability expenses | $	$ 0	$ 0	$ 0	$ (2,000,000)
Executives and Non Executives Employees
Share-based Compensation Arrangement by Share-based Payment Award
Limited partners capital account units held by BGC (in shares)	90,500,000		90,500,000
Non Distribution Earning Units
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting period			4 years
Unit granted during period (in shares)	5,200,000	3,800,000	19,800,000	9,900,000
Number of shares vested and converted (in shares)	2,600,000	2,500,000	8,900,000	7,400,000
Newmark Holdings, L.P.
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units exchange ratio	0.4545		0.4545
NEWMARK Group Inc Parent
Share-based Compensation Arrangement by Share-based Payment Award
Limited partners capital account units held by BGC (in shares)	16,700,196		16,700,196		14,277,213
Limited partnership units outstanding (in shares)	40,909,649		40,909,649		30,687,954	18,419,613
NEWMARK Group Inc Parent | Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units outstanding (in shares)	3,200,000		3,200,000		3,900,000
Exchangeable partnership units (in shares)	1,600,000		1,600,000		1,500,000
NEWMARK Group Inc Parent | Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Issuance of common stock and exchangeability expenses | $	$ 19,121,000	$ 33,330,000	$ 53,601,000	$ 69,188,000
BGC Holdings, L.P.
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units outstanding (in shares)	0		0		5,459,388	8,663,930
BGC Holdings, L.P. | Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units outstanding (in shares)					4,800,000
Exchangeable partnership units (in shares)					2,500,000
Maximum | NEWMARK Group Inc Parent | Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting period			7 years
Class A Common Stock
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units exchange ratio							1
Class A Common Stock | NEWMARK Group Inc Parent
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units exchange ratio	0.9243		0.9243
Newmark Equity Plan | Class A Common Stock | Maximum
Share-based Compensation Arrangement by Share-based Payment Award
Shares authorized to be delivered pursuant to awards granted (in shares)							400,000,000
Shares registered to be delivered pursuant to awards granted (in shares)							400,000,000