Number of offices acquired.

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Acquisitions - Additional Information (Detail) $ in Thousands					6 Months Ended	12 Months Ended
	Sep. 08, 2017 USD ($)	Sep. 07, 2017 Office	Jul. 26, 2017	Feb. 28, 2017	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Apr. 30, 2018 Office
Business Acquisition [Line Items]
Total consideration transferred					$ 10,000	$ 1,000,000
Business acquisition, contingent cash consideration					1,900	14,700
Additional goodwill recognized					6,626	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
Commercial Real Estate Consulting Firm [Member]
Business Acquisition [Line Items]
Business acquisition date			Jul. 26, 2017
Ownership percentage acquired			50.00%