Revenue from Contracts with Customers - Disaggregation of Revenue from Contracts with Customers (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	$ 23,274,086	$ 20,778,100	$ 18,152,507
Out of Scope of Topic 606 Revenue	620,005	561,988	476,280
Total revenue	23,894,091	21,340,088	18,628,787
Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	8,471,939	7,878,014	6,903,445
Out of Scope of Topic 606 Revenue	597,529	561,988	476,280
Total revenue	9,069,468	8,440,002	7,379,725
Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	14,164,001	12,365,362	10,791,963
Total revenue	14,164,001	12,365,362	10,791,963
Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	638,146	534,724	457,099
Out of Scope of Topic 606 Revenue	22,476
Total revenue	660,622	534,724	457,099
Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	14,164,001	12,365,362	10,791,963
Global Workplace Solutions [Member] \| Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	14,164,001	12,365,362	10,791,963
Advisory Leasing [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	3,269,993	3,080,117	2,592,203
Advisory Leasing [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	3,269,993	3,080,117	2,592,203
Advisory Sales [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	2,130,979	1,980,932	1,869,872
Advisory Sales [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	2,130,979	1,980,932	1,869,872
Property and Advisory Project Management [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	2,255,398	2,057,433	1,748,594
Property and Advisory Project Management [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	2,255,398	2,057,433	1,748,594
Valuation [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	630,399	598,806	556,008
Valuation [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	630,399	598,806	556,008
Commercial Mortgage Origination [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	154,227	136,534	117,009
Out of Scope of Topic 606 Revenue	421,736	402,814	338,590
Commercial Mortgage Origination [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	154,227	136,534	117,009
Out of Scope of Topic 606 Revenue	421,736	402,814	338,590
Loan Servicing [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	30,943	24,192	19,759
Out of Scope of Topic 606 Revenue	175,793	159,174	137,690
Loan Servicing [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	30,943	24,192	19,759
Out of Scope of Topic 606 Revenue	175,793	159,174	137,690
Investment Management [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	424,882	434,405	377,644
Investment Management [Member] \| Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	424,882	434,405	377,644
Development Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	213,264	100,319	79,455
Out of Scope of Topic 606 Revenue	22,476
Development Services [Member] \| Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	213,264	$ 100,319	$ 79,455
Out of Scope of Topic 606 Revenue	$ 22,476

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.

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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- Definition

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
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-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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-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

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