Compensation - Compensation Expense Related to Limited Partnership Units and Restricted Stock Units (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock and exchangeability expenses	$ 2,474	$ 661
Equity-based compensation and allocations of net income to limited partnership units and FPUs	12,914	13,871
Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock and exchangeability expenses	5,645	0
NEWMARK Group Inc Parent
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation and allocations of net income to limited partnership units and FPUs	12,914	13,871
NEWMARK Group Inc Parent | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units amortization	2,351	562
NEWMARK Group Inc Parent | Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock and exchangeability expenses	8,119	661
Allocations of net income	549	6,313
Limited partnership units amortization	$ 1,895	$ 6,335