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Cover Page - USD ($)
shares in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2024
Jan. 31, 2025
Jun. 30, 2024
Cover [Abstract]      
Amendment Flag false    
Document Type 10-K    
Document Fiscal Year Focus 2024    
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, 2024    
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    
Document Financial Statement Error Correction [Flag] false    
Entity Public Float     $ 49,339.2
Entity Common Stock, Shares Outstanding   254.7  
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 2025 Proxy Statement are incorporated by reference into this Form 10‑K in response to Part III to the extent described herein.

    
Auditor Opinion [Text Block]

Opinion on the Financial Statements

We have audited the accompanying consolidated balance sheets of Arthur J. Gallagher & Co. (the Company) as of December 31, 2024 and 2023, the related consolidated statements of earnings, comprehensive earnings, stockholders’ equity and cash flows for each of the three years in the period ended December 31, 2024, the related notes and the financial statement schedule listed in the Index at Item 15(2)(a) (collectively referred to as the “consolidated financial statements”). In our opinion, the consolidated financial statements present fairly, in all material respects, the financial position of the Company at December 31, 2024 and 2023, and the results of its operations and its cash flows for each of the three years in the period ended December 31, 2024, in conformity with U.S. generally accepted accounting principles.

We also have audited, in accordance with the standards of the Public Company Accounting Oversight Board (United States) (PCAOB), the Company's internal control over financial reporting as of December 31, 2024, based on criteria established in Internal Control-Integrated Framework issued by the Committee of Sponsoring Organizations of the Treadway Commission (2013 framework), and our report dated February 17, 2025 expressed an unqualified opinion thereon.