Revenue from Contracts with Customers - Disaggregation of Revenue from Contracts with Customers (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	$ 5,745,129	$ 4,999,172
Out of Scope of Topic 606 Revenue	144,039	136,338
Total revenue	5,889,168	5,135,510
Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	1,801,051	1,698,064
Out of Scope of Topic 606 Revenue	130,473	136,338
Total revenue	1,931,524	1,834,402
Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	3,746,174	3,165,915
Total revenue	3,746,174	3,165,915
Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	197,904	135,193
Out of Scope of Topic 606 Revenue	13,566
Total revenue	211,470	135,193
Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	3,746,174	3,165,915
Global Workplace Solutions [Member] \| Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	3,746,174	3,165,915
Advisory Leasing [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	607,111	622,640
Advisory Leasing [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	607,111	622,640
Advisory Sales [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	430,956	385,655
Advisory Sales [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	430,956	385,655
Property and Advisory Project Management [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	565,943	520,884
Property and Advisory Project Management [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	565,943	520,884
Valuation [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	147,752	138,326
Valuation [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	147,752	138,326
Commercial Mortgage Origination [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	37,880	23,573
Out of Scope of Topic 606 Revenue	85,202	97,306
Commercial Mortgage Origination [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	37,880	23,573
Out of Scope of Topic 606 Revenue	85,202	97,306
Loan Servicing [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	11,409	6,986
Out of Scope of Topic 606 Revenue	45,271	39,032
Loan Servicing [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	11,409	6,986
Out of Scope of Topic 606 Revenue	45,271	39,032
Investment Management [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	121,678	106,308
Investment Management [Member] \| Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	121,678	106,308
Development Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	76,226	28,885
Out of Scope of Topic 606 Revenue	13,566
Development Services [Member] \| Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	76,226	$ 28,885
Out of Scope of Topic 606 Revenue	$ 13,566

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No definition available.

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- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

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- Definition

Amount of revenue that is not accounted for under Topic 606.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(1))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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-Paragraph 22
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-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
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-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(1))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

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