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Document and Entity Information
Sep. 16, 2020
Cover [Abstract]  
Amendment Flag false
Entity Central Index Key 0000354190
Document Type 8-K
Document Period End Date Sep. 16, 2020
Entity Registrant Name ARTHUR J. GALLAGHER & CO.
Entity Incorporation State Country Code DE
Entity File Number 1-09761
Entity Tax Identification Number 36-2151613
Entity Address, Address Line One 2850 W. Golf Road
Entity Address, City or Town Rolling Meadows
Entity Address, State or Province IL
Entity Address, Postal Zip Code 60008-4050
City Area Code (630)
Local Phone Number 773-3800
Written Communications false
Soliciting Material false
Pre Commencement Tender Offer false
Pre Commencement Issuer Tender Offer false
Security 12b Title Common Stock, $1.00 par value
Trading Symbol AJG
Security Exchange Name NYSE
Entity Emerging Growth Company false