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Number of equity instruments other than options outstanding, including both vested and non-vested instruments.

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Number of shares purchased for issuance under share-based payment arrangement.

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Intrinsic value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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Number of options vested.

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Compensation - Narrative (Details)	3 Months Ended		12 Months Ended
Compensation - Narrative (Details)	Mar. 31, 2024 USD ($) shares	Mar. 31, 2023 USD ($) shares	Dec. 31, 2023 USD ($) shares	Dec. 31, 2022 shares	Dec. 13, 2017 shares
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units exchange ratio	0.4545
Unrecognized compensation expense \| $	$ 68,700,000
Compensation expense (benefit) \| $	$ 51,212,000	$ 35,407,000
Common stock, conversion for each restricted stock unit settled	1
Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Compensation expense (benefit) \| $	$ 0	$ 0
Non Distribution Earning Units
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting period	7 years
Unit granted during period (in shares)	1,200,000	1,100,000
Number of shares vested and converted (in shares)	2,500,000	2,500,000
Newmark Holdings, L.P.
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units exchange ratio	0.4545
NEWMARK Group Inc
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units outstanding (in shares)	46,233,230		42,376,966	30,687,954
NEWMARK Group Inc \| Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units outstanding (in shares)	3,000,000		3,100,000
Exchangeable partnership units (in shares)	1,400,000		1,500,000
NEWMARK Group Inc \| Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting period	7 years
Shares purchased for award (in shares)	3,200,000
Fair value of non-vested \| $	$ 46,000,000		$ 46,000,000
Unrecognized compensation expense \| $	$ 28,500,000
Cost not yet recognized, period for recognition	1 year 9 months
NEWMARK Group Inc \| Non Distribution Earning Units \| Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Shares purchased for award (in shares)	2,300,000		2,300,000
BGC Holdings, L.P.
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units outstanding (in shares)	0		0	5,459,388
BGC Holdings, L.P. \| Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units outstanding (in shares)	0
Maximum \| NEWMARK Group Inc \| 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
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units exchange ratio	0.9255
Newmark Equity Plan \| Class A Common Stock
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
Shares purchased for award (in shares)	100,100,000
Shares available for future awards (in shares)	299,900,000