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Acquisitions - Additional Information (Detail)					3 Months Ended	12 Months Ended
	Sep. 08, 2017 USD ($)	Sep. 07, 2017 Office	Jul. 26, 2017	Feb. 28, 2017	Mar. 31, 2018 USD ($) Acquisition	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Number of businesses acquired | Acquisition					0
Total consideration transferred						$ 1,000,000,000
Business acquisition, contingent cash consideration						14,700,000
Additional goodwill recognized					$ 0	91,300,000
Measurement period adjustments					$ (2,566,000)
Cantor [Member]
Business Acquisition [Line Items]
Noncontrolling interest			25.00%
BPF [Member]
Business Acquisition [Line Items]
Total consideration transferred	$ 875,000,000
Business acquisition price paid in units	$ 3,200,000
Business acquisition date	Sep. 08, 2017
Business acquisition, deferred tax asset						$ 108,600,000
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
Commercial Real Estate Consulting Firm [Member]
Business Acquisition [Line Items]
Business acquisition date			Jul. 26, 2017
Ownership percentage acquired			50.00%

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