Acquisitions - Additional Information (Detail) $ in Thousands	1 Months Ended						9 Months Ended	12 Months Ended
	Jun. 30, 2019	Apr. 30, 2019	Dec. 31, 2018 USD ($)	Sep. 30, 2018	Jul. 31, 2018 Office	Apr. 30, 2018 Office	Sep. 30, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Business acquisition, contingent cash consideration			$ 28,099				$ 29,774	$ 28,099
Additional goodwill recognized			$ 515,321				539,803	515,321	$ 477,532
Business acquisition, aggregate revenue contribution								28,500
MLG Commercial LLC [Member]
Business Acquisition [Line Items]
Business acquisition date		Apr. 30, 2019
ACRES [Member]
Business Acquisition [Line Items]
Business acquisition date	Jun. 30, 2019
Two Thousand Nineteen Acquisitions [Member]
Business Acquisition [Line Items]
Total consideration transferred							29,000
Business acquisition, contingent cash consideration							7,800
Additional goodwill recognized							24,857
Business acquisition, amount deductible for tax							18,800
Business acquisition, aggregate revenue contribution							$ 3,400
Two Thousand Nineteen Acquisitions [Member] | Newmark Holdings, L.P. [Member] | Limited Partnership Units [Member]
Business Acquisition [Line Items]
Business acquisition price paid (Partnership Units) | shares							233,779
Business acquisition, contingent non cash consideration, fair value							$ 1,900
Boston and Pittsburg Acquisitions [Member]
Business Acquisition [Line Items]
Business acquisition date						Apr. 30, 2018
Number of offices acquired | Office						2
Denver and Pasadena Acquisitions [Member]
Business Acquisition [Line Items]
Business acquisition date					Jul. 31, 2018
Number of offices acquired | Office					2
RKF Retail Holdings LLC Acquisitions [Member]
Business Acquisition [Line Items]
Business acquisition date				Sep. 30, 2018
MiT National Land Services LLC Acquisitions [Member]
Business Acquisition [Line Items]
Business acquisition date			Dec. 31, 2018
Two Thousand Eighteen Acquisitions [Member]
Business Acquisition [Line Items]
Total consideration transferred								62,900
Business acquisition, contingent cash consideration			$ 8,600					8,600
Additional goodwill recognized			42,188					42,188
Business acquisition, amount deductible for tax			28,600					$ 28,600
Two Thousand Eighteen Acquisitions [Member] | Restricted Stock [Member]
Business Acquisition [Line Items]
Business acquisition price paid (Partnership Units) | shares								216,900
Business acquisition, contingent non cash consideration, fair value			3,100					$ 3,100
Two Thousand Eighteen Acquisitions [Member] | Newmark Holdings, L.P. [Member] | Limited Partnership Units [Member]
Business Acquisition [Line Items]
Business acquisition price paid (Partnership Units) | shares								465,316
Business acquisition, contingent non cash consideration, fair value			$ 6,200					$ 6,200