Cover - USD ($)	12 Months Ended
	Dec. 31, 2021	Feb. 17, 2022	Jun. 30, 2021
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	1-7933
Entity Registrant Name	Aon plc
Entity Incorporation, State or Country Code	L2
Entity Tax Identification Number	98-1539969
Entity Address, Address Line One	Metropolitan Building, James Joyce Street
Entity Address, City or Town	Dublin 1
Entity Address, Country	IE
Entity Address, Postal Zip Code	D01 K0Y8
City Area Code	1
Local Phone Number	266 6000
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 53,867,751,208
Entity Common Stock, Shares Outstanding		213,944,460
Documents Incorporated by Reference	Portions of the registrant’s proxy statement for its 2022 Annual General Meeting of Shareholders are incorporated by reference in this report in response to Part III, Items 10, 11, 12, 13, and 14.
Entity Central Index Key	0000315293
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Class A Ordinary Shares, $0.01 nominal value
Entity Information [Line Items]
Title of 12(b) Security	Class A Ordinary Shares, $0.01 nominal value
Trading Symbol	AON
Security Exchange Name	NYSE
Guarantees of Aon plc’s 4.00% Senior Notes due 2023
Entity Information [Line Items]
Title of 12(b) Security	Guarantees of Aon plc’s 4.00% Senior Notes due 2023
Trading Symbol	AON23
Security Exchange Name	NYSE
Guarantees of Aon plc’s 3.50% Senior Notes due 2024
Entity Information [Line Items]
Title of 12(b) Security	Guarantees of Aon plc’s 3.50% Senior Notes due 2024
Trading Symbol	AON24
Security Exchange Name	NYSE
Guarantees of Aon plc’s 3.875% Senior Notes due 2025
Entity Information [Line Items]
Title of 12(b) Security	Guarantees of Aon plc’s 3.875% Senior Notes due 2025
Trading Symbol	AON25
Security Exchange Name	NYSE
Guarantees of Aon plc’s 2.875% Senior Notes due 2026
Entity Information [Line Items]
Title of 12(b) Security	Guarantees of Aon plc’s 2.875% Senior Notes due 2026
Trading Symbol	AON26
Security Exchange Name	NYSE
Guarantees of Aon Corporation and Aon Global Holdings plc’s 2.05% Senior Notes due 2031
Entity Information [Line Items]
Title of 12(b) Security	Guarantees of Aon Corporation and Aon Global Holdings plc’s 2.05% Senior Notes due 2031
Trading Symbol	AON31
Security Exchange Name	NYSE
Guarantees of Aon Corporation and Aon Global Holdings plc’s 2.60% Senior Notes due 2031
Entity Information [Line Items]
Title of 12(b) Security	Guarantees of Aon Corporation and Aon Global Holdings plc’s 2.60% Senior Notes due 2031
Trading Symbol	AON31A
Security Exchange Name	NYSE
Guarantees of Aon plc’s 4.25% Senior Notes due 2042
Entity Information [Line Items]
Title of 12(b) Security	Guarantees of Aon plc’s 4.25% Senior Notes due 2042
Trading Symbol	AON42
Security Exchange Name	NYSE
Guarantees of Aon plc’s 4.45% Senior Notes due 2043
Entity Information [Line Items]
Title of 12(b) Security	Guarantees of Aon plc’s 4.45% Senior Notes due 2043
Trading Symbol	AON43
Security Exchange Name	NYSE
Guarantees of Aon plc’s 4.60% Senior Notes due 2044
Entity Information [Line Items]
Title of 12(b) Security	Guarantees of Aon plc’s 4.60% Senior Notes due 2044
Trading Symbol	AON44
Security Exchange Name	NYSE
Guarantees of Aon plc’s 4.75% Senior Notes due 2045
Entity Information [Line Items]
Title of 12(b) Security	Guarantees of Aon plc’s 4.75% Senior Notes due 2045
Trading Symbol	AON45
Security Exchange Name	NYSE
Guarantees of Aon Corporation and Aon Global Holdings plc’s 2.90% Senior Notes due 2051
Entity Information [Line Items]
Title of 12(b) Security	Guarantees of Aon Corporation and Aon Global Holdings plc’s 2.90% Senior Notes due 2051
Trading Symbol	AON51
Security Exchange Name	NYSE