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Acquisitions - Additional Information (Detail) $ in Thousands							9 Months Ended	12 Months Ended
	Sep. 04, 2018	Jul. 20, 2018	Sep. 08, 2017 USD ($)	Sep. 07, 2017 Office	Jul. 26, 2017	Feb. 28, 2017	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jul. 31, 2018 Office	Apr. 30, 2018 Office
Business Acquisition [Line Items]
Total consideration transferred							$ 61,200	$ 1,000,000
Business acquisition, contingent cash consideration							17,000	14,700
Additional goodwill recognized							38,439	91,300
Measurement period adjustments							$ (2,500)
Cantor [Member]
Business Acquisition [Line Items]
Noncontrolling interest					25.00%
BPF [Member]
Business Acquisition [Line Items]
Total consideration transferred			$ 875,000
Business acquisition price paid in units			$ 3,200
Business acquisition date			Sep. 08, 2017
Business acquisition, deferred tax asset								$ 108,600
Besso [Member]
Business Acquisition [Line Items]
Business acquisition date						Feb. 28, 2017
Integra Realty Resources [Member]
Business Acquisition [Line Items]
Business acquisition date				Sep. 07, 2017
Number of offices acquired \| Office				6					2	2
Commercial Real Estate Consulting Firm [Member]
Business Acquisition [Line Items]
Business acquisition date					Jul. 26, 2017
Ownership percentage acquired					50.00%
Jackson & Cooksey, Inc. [Member]
Business Acquisition [Line Items]
Business acquisition date		Jul. 20, 2018
RKF Retail Holdings,LLC [Member]
Business Acquisition [Line Items]
Business acquisition date	Sep. 04, 2018