Nature of Operations - Additional Information (Detail)	12 Months Ended
Dec. 31, 2017 Employee Office
Schedule Of Description Of Business [Line Items]
Number of offices \| Office	450
Minimum [Member]
Schedule Of Description Of Business [Line Items]
Number of employees \| Employee	80,000

The approximate number of offices in which the entity operates worldwide.

No definition available.

Name:	cbg_NumberOfOffices
Namespace Prefix:	cbg_
Data Type:	xbrli:positiveIntegerItemType
Balance Type:	na
Period Type:	duration