v3.26.3
Cover Page - shares
shares in Millions
9 Months Ended
Sep. 30, 2026
Oct. 30, 2026
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2026  
Document Transition Report false  
Entity File Number 1-09761  
Entity Registrant Name ARTHUR J. GALLAGHER & CO.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 36-2151613  
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  
Title of 12(b) Security Common Stock, par value $1.00 per share  
Trading Symbol AJG  
Security Exchange Name NYSE  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   256.4
Entity Central Index Key 0000354190  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q3  
Amendment Flag false