| Document and Entity Information | May 12, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | Arthur J. Gallagher & Co. |
| Amendment Flag | false |
| Entity Central Index Key | 0000354190 |
| Document Type | 8-K |
| Document Period End Date | May 12, 2021 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 1-09761 |
| Entity Tax Identification Number | 36-2151613 |
| Entity Address, Address Line One | 2850 W. 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $1.00 par value |
| Trading Symbol | AJG |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
