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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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