Cover Page	6 Months Ended
	Jun. 30, 2022 shares
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Fiscal Year Focus	2022
Document Period End Date	Jun. 30, 2022
Document Fiscal Period Focus	Q2
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	210,336,000
Document Quarterly Report	true