FacilitySource Acquisition - Certain Adjustments to Unaudited Pro Forma Results (Detail) - FacilitySource [Member] - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Business Acquisition [Line Items]
Depreciation expense	$ 1,201	$ 1,235	$ 2,642	$ 2,469
Amortization expense	1,731	2,078	3,809	4,155
Interest expense	$ 1,224	$ 1,525	$ 2,748	$ 3,049

The amount of incremental amortization expense that would have been recognized for the period if the business combination or combinations had been completed at the beginning of a period.

No definition available.

Name:	cbg_BusinessAcquisitionsIncrementalAmortizationExpense
Namespace Prefix:	cbg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration