Cover Page - USD ($)

12 Months Ended

Dec. 31, 2020

Jan. 31, 2021

Jun. 30, 2020

Cover [Abstract]

Amendment Flag

false

Document Type

10-K

Document Fiscal Year Focus

2020

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, 2020

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

$ 16,249,616,000

Entity Common Stock, Shares Outstanding

193,740,000

Documents Incorporated by Reference

Documents incorporated by reference: Portions of Arthur J. Gallagher & Co.’s definitive 2021 Proxy Statement are incorporated by reference into this Form 10‑K in response to Part III to the extent described herein.