Compensation - Summary of Activity of Number of Share Equivalent Limited Partnership Units and Post IPO Grants (Detail) - Newmark Holdings, L.P. [Member] - Share Equivalent Limited Partnership Units [Member] - shares	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Units, Balance outstanding at beginning of period	44,733,487	29,413,143
Number of Units, Granted	11,919,128	19,141,943
Number of Units, Redeemed/exchanged units	(1,533,472)	(3,793,351)
Number of Units, Forfeited units		(28,248)
Number of Units, Forfeited units/Other units	4,814,499
Number of Units, Balance outstanding at end of period	59,933,642	44,733,487