Compensation - Compensation Expense Related to Limited Partnership Units and Restricted Stock Units (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award
Issuance of common stock and exchangeability expenses	$ 7	$ 2,559	$ 2,481	$ 3,220
Equity-based compensation and allocations of net income to limited partnership units and FPUs			23,774	53,224
Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Issuance of common stock and exchangeability expenses	299	18,952	5,944	18,952
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	10,981	39,353	23,895	53,224
NEWMARK Group Inc Parent | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award
Limited partnership units amortization	3,560	1,197	5,911	1,760
NEWMARK Group Inc Parent | Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award
Issuance of common stock and exchangeability expenses	306	21,511	8,425	22,172
Allocations of net income to limited partnership units and FPUs	1,104	11,601	1,653	17,915
Limited partnership units amortization	$ 6,011	$ 5,044	$ 7,906	$ 11,377