XML 32 R1.htm IDEA: XBRL DOCUMENT v3.19.3.a.u2
Cover Page - USD ($)
 12 Months Ended
Dec. 31, 2019
Jan. 31, 2020
Jun. 30, 2018
Cover [Abstract]      
Amendment Flag false    
Document Type 10-K    
Document Fiscal Year Focus 2019    
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, 2019    
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    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Public Float     $ 14,245,000
Entity Common Stock, Shares Outstanding   188,247,000