Compensation - Compensation Expense Related to Limited Partnership Units and Restricted Stock Units (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award
Issuance of common stock and exchangeability expenses	$ 125	$ 0	$ 2,606	$ 2,719
Equity-based compensation and allocations of net income to limited partnership units and FPUs			74,544	109,872
Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Issuance of common stock and exchangeability expenses	13,118	17,499	19,062	36,952
NEWMARK Group Inc Parent
Share-based Compensation Arrangement by Share-based Payment Award
Equity-based compensation and allocations of net income to limited partnership units and FPUs	50,770	56,647	74,544	109,871
NEWMARK Group Inc Parent | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units amortization	3,404	1,413	9,315	3,172
NEWMARK Group Inc Parent | Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Issuance of common stock and exchangeability expenses	13,243	17,499	21,668	39,671
Allocations of net income to limited partnership units and FPUs	29,221	32,496	30,753	50,410
Limited partnership units amortization	$ 4,902	$ 5,239	$ 12,808	$ 16,618