FacilitySource Acquisition - Certain Adjustments to Unaudited Pro Forma Results (Detail) - FacilitySource [Member] - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Depreciation expense	$ 1,253	$ 3,054	$ 3,298
Amortization expense	1,019	2,190	2,190
Interest expense	$ 2,748	$ 6,098	$ 6,098

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