Number of offices acquired.

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Value of equity interests (such as common shares, preferred shares, or partnership interest) issued or issuable to acquire the entity.

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Number of shares of equity interests issued or issuable to acquire entity.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The amount of goodwill arising from a business combination that is expected to be deductible for tax purposes.

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Total revenue reported by the acquired entity for its previous full fiscal year before the business acquisition.

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Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

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Amount of liability recognized arising from contingent consideration in a business combination, expected to be settled beyond one year or the normal operating cycle, if longer.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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The number of businesses acquired by the entity during the period.

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Acquisitions - Additional Information (Detail) $ in Thousands				3 Months Ended	12 Months Ended
	Sep. 08, 2017 USD ($)	Sep. 07, 2017 Office	Jul. 26, 2017	Mar. 31, 2018 USD ($) Business	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Number of businesses acquired \| Business				0
Total consideration transferred					$ 55,600
Business acquisition, contingent cash consideration				$ 15,971	17,207
Additional goodwill recognized				$ 474,990	477,532	$ 412,846
Business acquisition, amount deductible for tax					45,400
Business acquisition, aggregate revenue contribution					$ 13,100
BGC's Holding Partnership Units [Member]
Business Acquisition [Line Items]
Business acquisition price paid (Partnership Units) \| shares					477,169
Business acquisition, contingent non cash consideration, fair value					$ 5,000
Business acquisition, contingent cash consideration					1,300
Additional goodwill recognized					64,300
Cantor [Member]
Business Acquisition [Line Items]
Noncontrolling interest, ownership percentage			25.00%
BPF [Member]
Business Acquisition [Line Items]
Business acquisition date	Sep. 08, 2017
Ownership percentage acquired	100.00%
Business acquisition, deferred tax asset					$ 108,600
Total consideration transferred	$ 875,000
Commercial Real Estate Consulting Firm [Member]
Business Acquisition [Line Items]
Business acquisition date			Jul. 26, 2017
Ownership percentage acquired			50.00%
Integra Realty Resources [Member]
Business Acquisition [Line Items]
Business acquisition date		Sep. 07, 2017
Number of offices acquired \| Office		6