Compensation - Activity Associated with Limited Partnership Units (Detail) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
NEWMARK Group Inc Parent
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding
Number of Units, Balance outstanding at beginning of period (in shares)	60,800,852	44,733,487
Number of Units, Granted (in shares)	12,569,298	13,813,204
Number of units, Redeemed/exchanged units (in shares)	(6,660,984)	(2,487,885)
Number of Units, Forfeited units (in shares)	(82,981)	4,742,046
Number of Units, Balance outstanding at end of period (in shares)	66,626,185	60,800,852
Exchangeable Units (units)	9,906,763	10,108,598
BGC Holdings, L.P.
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding
Number of Units, Balance outstanding at beginning of period (in shares)	56,053,701	61,870,969
Number of Units, Granted (in shares)	1,071,612	319,586
Number of units, Redeemed/exchanged units (in shares)	(2,657,901)	(3,938,134)
Number of Units, Forfeited units (in shares)	(45,410)	(2,198,720)
Number of Units, Balance outstanding at end of period (in shares)	54,422,002	56,053,701
Exchangeable Units (units)	24,863,107	24,692,695
Regular units | NEWMARK Group Inc Parent
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding
Number of Units, Balance outstanding at end of period (in shares)	62,157,822
Regular units | BGC Holdings, L.P.
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding
Number of Units, Balance outstanding at end of period (in shares)	52,833,075
Preferred Units | NEWMARK Group Inc Parent
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding
Number of Units, Balance outstanding at end of period (in shares)	4,468,363
Preferred Units | BGC Holdings, L.P.
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding
Number of Units, Balance outstanding at end of period (in shares)	1,588,927