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Document And Entity Information
 12 Months Ended
Dec. 31, 2017
shares
Document Information [Line Items]  
Entity Registrant Name Colliers International Group Inc.
Entity Central Index Key 0000913353
Trading Symbol cigi
Current Fiscal Year End Date --12-31
Entity Filer Category Large Accelerated Filer
Entity Current Reporting Status Yes
Entity Voluntary Filers No
Entity Well-known Seasoned Issuer No
Document Type 40-F
Document Period End Date Dec. 31, 2017
Document Fiscal Year Focus 2017
Document Fiscal Period Focus FY
Amendment Flag false
Subordinate Voting Shares [Member]  
Document Information [Line Items]  
Entity Common Stock, Shares Outstanding (in shares) 37,608,467
Multiple Voting Shares [Member]  
Document Information [Line Items]  
Entity Common Stock, Shares Outstanding (in shares) 1,325,694