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Document and Entity Information
 9 Months Ended
Sep. 30, 2017
shares
Document And Entity Information [Abstract]  
Document Type 10-Q
Amendment Flag false
Document Period End Date Sep. 30, 2017
Document Fiscal Year Focus 2017
Document Fiscal Period Focus Q3
Trading Symbol AJG
Entity Registrant Name GALLAGHER ARTHUR J & CO
Entity Central Index Key 0000354190
Current Fiscal Year End Date --12-31
Entity Filer Category Large Accelerated Filer
Entity Common Stock, Shares Outstanding 180,799,000