Compensation - Compensation Expense Related to Limited Partnership Units and Restricted Stock Units (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock and exchangeability expenses	$ 35,499	$ 142,333	$ 99,435
Equity-based compensation and allocations of net income to limited partnership units and FPUs	258,836	224,644	10,000
Limited Partnership Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock and exchangeability expenses	146,215	37,000	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	258,836	224,644	147,139
NEWMARK Group Inc Parent | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Limited partnership units amortization	5,204	1,787	1,212
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	181,714	179,333	99,435
Allocations of net income	50,410	51,462	25,222
Limited partnership units amortization	$ 21,508	$ (7,938)	$ 21,270