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Note 1 - Description of the Business (Details Textual)	Dec. 31, 2017
Number of Countries in which Entity Operates	35
Number of Geographic Regions in which Entity Operates	3
Colliers and Its Affiliates and Franchisees [Member]
Number of Countries in which Entity Operates	69

X

- Definition

The number of geographic regions in which the entity operates as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	cigi_NumberOfGeographicRegionsInWhichEntityOperates
Namespace Prefix:	cigi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Definition

The number of countries in which the entity operates as of balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfCountriesInWhichEntityOperates
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	dei_LegalEntityAxis=cigi_ColliersAndItsAffiliatesAndFranchiseesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: