Revenue from Contracts with Customers - Disaggregation of Revenue from Contracts with Customers (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Disaggregation Of Revenue [Line Items]
Total revenue	$ 4,999,172	$ 4,554,403
Total revenue	136,338	119,549
Total revenue	5,135,510	4,673,952
Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	1,698,064	1,579,885
Total revenue	136,338	119,549
Total revenue	1,834,402	1,699,434
Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	3,165,915	2,827,503
Total revenue	3,165,915	2,827,503
Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	135,193	147,015
Total revenue	135,193	147,015
Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	3,165,915	2,827,503
Global Workplace Solutions [Member] \| Global Workplace Solutions [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	3,165,915	2,827,503
Advisory Leasing [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	622,640	517,509
Advisory Leasing [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	622,640	517,509
Advisory Sales [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	385,655	413,732
Advisory Sales [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	385,655	413,732
Property and Advisory Project Management [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	520,884	484,782
Property and Advisory Project Management [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	520,884	484,782
Valuation [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	138,326	134,163
Valuation [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	138,326	134,163
Commercial Mortgage Origination [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	23,573	24,282
Total revenue	97,306	83,152
Commercial Mortgage Origination [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	23,573	24,282
Total revenue	97,306	83,152
Loan Servicing [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	6,986	5,417
Total revenue	39,032	36,397
Loan Servicing [Member] \| Advisory Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	6,986	5,417
Total revenue	39,032	36,397
Investment Management [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	106,308	123,690
Investment Management [Member] \| Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	106,308	123,690
Development Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	28,885	23,325
Development Services [Member] \| Real Estate Investments [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	$ 28,885	$ 23,325