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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

X
- Definition The approximate number of offices in which the entity operates worldwide. + References No definition available. + Details
Name: | cbg_NumberOfOffices
Namespace Prefix: | cbg_
Data Type: | xbrli:positiveIntegerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Schedule Of Description Of Business [Line Items] + References No definition available. + Details
Name: | cbg_ScheduleOfDescriptionOfBusinessLineItems
Namespace Prefix: | cbg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of persons employed by the Entity + References No definition available. + Details
Name: | dei_EntityNumberOfEmployees
Namespace Prefix: | dei_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |