Derivatives - Fair Value of Derivative Contracts (Parenthetical) (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Derivative contract, Assets		$ 68,696		$ 68,696		$ 92,528
Valuation of derivative asset				37,181	$ (6,327)
Derivative contract, Notional Amounts		2,020,123		2,020,123		1,814,834
Forward Contracts [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Derivative contract, Assets	[1]	42,005		42,005		85,796
Derivative contract, Notional Amounts	[2]	1,531,459		1,531,459		1,574,114
Forward Contracts [Member] | NASDAQ [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Derivative contract, Assets		40,400		40,400		77,600
Valuation of derivative asset		(8,200)	$ 9,100	(37,200)	$ 6,300
Derivative contract, Notional Amounts		$ 361,000		$ 361,000		$ 361,000

[1] Included in Forwards is $40.4 million and $77.6 million of the Nasdaq Forwards as of September 30, 2019 and December 31, 2018, respectively (See Note 1 — “Organization and Basis of Presentation”). Unrealized gains/(losses) for the change in the fair value of the Nasdaq Forwards for the three months ended September 30, 2019 and 2018 was $(8.2) million and $9.1 million, respectively and $(37.2) million and $6.3 million for the nine months ended September 30, 2019 and 2018, respectively.

[2] Notional amounts represent the sum of gross long and short derivative contracts, an indication of the volume of Newmark’s derivative activity, and does not represent anticipated losses. Included in the notional amounts of forwards is $361.0 million for the Nasdaq Forwards as of September 30, 2019 and December 31, 2018.