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Employee Benefit Plans - Additional Information (Detail)			3 Months Ended	12 Months Ended
	Dec. 15, 2017 shares	Dec. 01, 2017 shares	May 17, 2019 shares	Dec. 31, 2019 USD ($) Pension_Plan shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	May 19, 2017 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Defined benefit plan \| $				$ 309,400,000	$ 363,600,000	$ 286,500,000
Plan name				CBRE 401(k) Plan
Defined Contribution Plan, Sponsor Location [Extensible List]				us-gaap:DomesticPlanMember
Company contribution percent per year employed				20.00%
Percent of annual compensation match percentage					50.00%	50.00%
Percent of annual compensation to be matched					6.00%	6.00%
Defined benefit plan, annual compensation expense maximum \| $					$ 150,000	$ 150,000
Defined contribution plan, expenses recognized \| $				$ 59,900,000	46,300,000	$ 38,800,000
Percent of 401(k) that can be invested in common stock				25.00%
Number of share held as investment under 401(k) Plan				1,300,000
Defined Benefit Plan, Type [Extensible List]				us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]				us-gaap:ForeignPlanMember
Number of contributory defined benefit pension plans in the United Kingdom \| Pension_Plan				2
Fair value of plan assets \| $				$ 331,400,000	274,400,000
Benefit obligation \| $				439,400,000	387,400,000
Funded status \| $				108,000,000.0	113,000,000.0
Unamortized actuarial loss \| $				$ 191,600,000	$ 192,700,000
Annual Base Salary of Less than $100,000 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of annual compensation match percentage				67.00%
Percent of annual compensation to be matched				6.00%
Defined benefit plan, annual compensation expense maximum \| $				$ 100,000
Annual Base Salary of $100,000 or Up to $150,000 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of annual compensation match percentage				50.00%
Percent of annual compensation to be matched				6.00%
Defined benefit plan, annual compensation expense minimum \| $				$ 100,000,000
Defined benefit plan, annual compensation expense maximum \| $				$ 150,000
Time Based Vesting [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Time-vesting awards, portion to be vested per year				25.00%
Vesting period				4 years
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee subscription rate				75.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee subscription rate				1.00%
Restricted Stock Units [Member] \| Time Based Vesting [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant				1,493,788	1,332,085	1,466,986
Restricted Stock Units [Member] \| Time Based Vesting [Member] \| Senior Brokers [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Granted, Shares / Units	127,160
Restricted Stock Units [Member] \| Maximum [Member] \| Performance Based Vesting [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant				888,726	1,014,269	1,458,033
Non-Vested Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Shares / Units				2,000,977	2,023,266	5,152,082
Stock options, compensation expense \| $				$ 127,700,000	$ 128,200,000	$ 93,100,000
Stock options, unrecognized estimated compensation cost \| $				$ 173,800,000
Weighted average period of recognition				2 years 10 months 24 days
2017 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unissued common stock available for issuance							0
Shares granted in period under equity incentive plan			189,499
Shares cancelled under equity incentive plan				234,242
2017 Equity Incentive Plan [Member] \| Restricted Stock Units [Member] \| Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of trading days required for purposes of measuring shareholder return based on average closing price of common stock immediately preceding grant date		60 days
2017 Equity Incentive Plan [Member] \| Restricted Stock Units [Member] \| Time Based Vesting [Member] \| Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant		939,605		55,973	55,973
Share based compensation arrangement by share based payment percentage of target restricted stock unit		33.30%
2017 Equity Incentive Plan [Member] \| Restricted Stock Units [Member] \| Total Shareholder Return [Member] \| Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity incentive plan termination date		Dec. 01, 2023
Share based compensation arrangement by share based payment percentage of target restricted stock unit		33.30%
Share-based compensation arrangement by share-based payment award, measurement period		6 years
2017 Equity Incentive Plan [Member] \| Restricted Stock Units [Member] \| EPS [Member] \| Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity incentive plan termination date		Dec. 31, 2023
Share based compensation arrangement by share based payment percentage of target restricted stock unit		33.30%
Share-based compensation arrangement by share-based payment award, measurement period		6 years
2017 Equity Incentive Plan [Member] \| Restricted Stock Units [Member] \| Maximum [Member] \| Performance Based Vesting [Member] \| Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant		3,288,618		195,907	195,907
2012 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant				2,166,537
2019 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unissued common stock available for issuance							0
Equity incentive plan termination date				Mar. 01, 2029
2019 Equity Incentive Plan [Member] \| Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant				7,538,712
Class A Common Stock [Member] \| 2017 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance				5,736,560
Class A Common Stock [Member] \| 2019 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant				9,944,743
Shares reserved for future issuance			9,900,000