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Cover Page
 3 Months Ended
Mar. 31, 2025
shares
Cover [Abstract]  
Document Type 10-Q
Amendment Flag false
Document Fiscal Year Focus 2025
Document Period End Date Mar. 31, 2025
Document Fiscal Period Focus Q1
Entity Interactive Data Current Yes
Entity Central Index Key 0000354190
Current Fiscal Year End Date --12-31
Entity Registrant Name ARTHUR J. GALLAGHER & CO.
Entity File Number 1-09761
Document Transition Report false
Entity Tax Identification Number 36-2151613
Entity Incorporation, State or Country Code DE
Entity Current Reporting Status Yes
Entity Filer Category Large Accelerated Filer
Entity Small Business false
Entity Emerging Growth Company false
Entity Address, Address Line One 2850 Golf Road
Entity Address, City or Town Rolling Meadows
Entity Address, State or Province IL
Entity Address, Postal Zip Code 60008
City Area Code 630
Local Phone Number 773-3800
Entity Shell Company false
Trading Symbol AJG
Security Exchange Name NYSE
Title of 12(b) Security Common Stock, par value $1.00 per share
Entity Common Stock, Shares Outstanding 256,100,000
Document Quarterly Report true