Document and Entity Information
9 Months Ended
Sep. 30, 2018
Document And Entity Information [Abstract]  
Document Type 10-Q
Amendment Flag false
Document Period End Date Sep. 30, 2018
Document Fiscal Year Focus 2018
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
Smaller Reporting Company false
Emerging Growth Company false
Entity Common Stock, Shares Outstanding 183,652,000