Revenue from Contracts with Customers - Disaggregation of Revenue from Contracts with Customers (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	$ 5,760,025	$ 5,118,285	$ 16,322,726	$ 14,652,748
Out of Scope of Topic 606 Revenue	165,076	142,669	451,958	393,592
Total revenue	5,925,101	5,260,954	16,774,684	15,046,340
Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	2,075,636	1,916,690	5,802,114	5,324,307
Out of Scope of Topic 606 Revenue	165,076	142,669	451,958	393,592
Total revenue	2,240,712	2,059,359	6,254,072	5,717,899
Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	3,555,176	3,082,781	10,106,543	8,945,257
Total revenue	3,555,176	3,082,781	10,106,543	8,945,257
Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	129,213	118,814	414,069	383,184
Total revenue	129,213	118,814	414,069	383,184
Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	3,555,176	3,082,781	10,106,543	8,945,257
Global Workplace Solutions [Member] \| Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	3,555,176	3,082,781	10,106,543	8,945,257
Advisory Leasing [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	781,246	751,793	2,221,674	1,956,688
Advisory Leasing [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	781,246	751,793	2,221,674	1,956,688
Advisory Sales [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	526,104	486,398	1,378,317	1,357,506
Advisory Sales [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	526,104	486,398	1,378,317	1,357,506
Property and Advisory Project Management [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	558,039	498,388	1,634,745	1,485,371
Property and Advisory Project Management [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	558,039	498,388	1,634,745	1,485,371
Valuation [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	154,861	143,814	442,238	425,374
Valuation [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	154,861	143,814	442,238	425,374
Commercial Mortgage Origination [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	46,787	30,939	102,471	83,077
Out of Scope of Topic 606 Revenue	117,052	101,561	322,246	277,695
Commercial Mortgage Origination [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	46,787	30,939	102,471	83,077
Out of Scope of Topic 606 Revenue	117,052	101,561	322,246	277,695
Loan Servicing [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	8,599	5,358	22,669	16,291
Out of Scope of Topic 606 Revenue	48,024	41,108	129,712	115,897
Loan Servicing [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	8,599	5,358	22,669	16,291
Out of Scope of Topic 606 Revenue	48,024	41,108	129,712	115,897
Investment Management [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	104,927	93,061	312,881	315,698
Investment Management [Member] \| Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	104,927	93,061	312,881	315,698
Development Services [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	24,286	25,753	101,188	67,486
Development Services [Member] \| Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Topic 606 Revenue	$ 24,286	$ 25,753	$ 101,188	$ 67,486

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
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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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