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Cover Page - USD ($)
 12 Months Ended
Dec. 31, 2021
Jan. 31, 2022
Jun. 30, 2021
Cover [Abstract]      
Amendment Flag false    
Document Type 10-K    
Document Fiscal Year Focus 2021    
Entity Interactive Data Current Yes    
Document Fiscal Period Focus FY    
Document Annual Report true    
Entity Central Index Key 0000354190    
Document Transition Report false    
Current Fiscal Year End Date --12-31    
Document Period End Date Dec. 31, 2021    
Entity Registrant Name ARTHUR J. GALLAGHER & CO.    
Entity File Number 1-09761    
Entity Tax Identification Number 36-2151613    
Entity Incorporation, State or Country Code DE    
Entity Current Reporting Status Yes    
Entity Shell Company false    
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 60008-4050    
City Area Code 630    
Local Phone Number 773-3800    
Trading Symbol AJG    
Security Exchange Name NYSE    
Title of 12(b) Security Common Stock, par value $1.00 per share    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
ICFR Auditor Attestation Flag true    
Entity Public Float     $ 25,186,468,000
Entity Common Stock, Shares Outstanding   208,542,000  
Auditor Name Ernst & Young LLP    
Auditor Location Chicago, Illinois    
Auditor Firm ID 42    
Documents Incorporated by Reference

Documents incorporated by reference: Portions of Arthur J. Gallagher & Co.’s definitive 2022 Proxy Statement are incorporated by reference into this Form 10‑K in response to Part III to the extent described herein.