Revenue from Contracts with Customers - Disaggregation of Revenue from Contracts with Customers (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	$ 5,252,241	$ 5,563,529	$ 10,997,369	$ 10,562,701
Out of Scope of Topic 606 Revenue	129,143	150,544	273,183	286,882
Total revenue	5,381,384	5,714,073	11,270,552	10,849,583
Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	1,424,565	2,028,414	3,225,616	3,726,478
Out of Scope of Topic 606 Revenue	128,364	150,544	258,837	286,882
Total revenue	1,552,929	2,178,958	3,484,453	4,013,360
Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	3,666,844	3,385,452	7,413,018	6,551,367
Total revenue	3,666,844	3,385,452	7,413,018	6,551,367
Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	160,832	149,663	358,735	284,856
Out of Scope of Topic 606 Revenue	779		14,346
Total revenue	161,611	149,663	373,081	284,856
Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	3,666,844	3,385,452	7,413,018	6,551,367
Global Workplace Solutions [Member] \| Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	3,666,844	3,385,452	7,413,018	6,551,367
Advisory Leasing [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	510,124	817,788	1,117,235	1,440,428
Advisory Leasing [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	510,124	817,788	1,117,235	1,440,428
Advisory Sales [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	241,247	466,558	672,203	852,213
Advisory Sales [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	241,247	466,558	672,203	852,213
Property and Advisory Project Management [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	512,218	555,822	1,078,161	1,076,706
Property and Advisory Project Management [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	512,218	555,822	1,078,161	1,076,706
Valuation [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	131,845	149,051	279,597	287,377
Valuation [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	131,845	149,051	279,597	287,377
Commercial Mortgage Origination [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	20,110	32,111	57,990	55,684
Out of Scope of Topic 606 Revenue	80,335	107,888	165,537	205,194
Commercial Mortgage Origination [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	20,110	32,111	57,990	55,684
Out of Scope of Topic 606 Revenue	80,335	107,888	165,537	205,194
Loan Servicing [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	9,021	7,084	20,430	14,070
Out of Scope of Topic 606 Revenue	48,029	42,656	93,300	81,688
Loan Servicing [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	9,021	7,084	20,430	14,070
Out of Scope of Topic 606 Revenue	48,029	42,656	93,300	81,688
Investment Management [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	103,132	101,646	224,810	207,954
Investment Management [Member] \| Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	103,132	101,646	224,810	207,954
Development Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	57,700	48,017	133,925	76,902
Out of Scope of Topic 606 Revenue	779		14,346
Development Services [Member] \| Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	57,700	$ 48,017	133,925	$ 76,902
Out of Scope of Topic 606 Revenue	$ 779		$ 14,346

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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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
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-Section 50
-Paragraph 32
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-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599

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

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-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

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