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Nature of Operations - Additional Information (Detail) - Minimum [Member]	12 Months Ended
	Dec. 31, 2018 Employee Office
Schedule Of Description Of Business [Line Items]
Number of offices | Office	480
Number of employees | Employee	90,000

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:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: