Cover - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 13, 2020	Jun. 28, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
Document Transition Report	false
Entity File Number	1-7933
Entity Registrant Name	Aon plc
Entity Incorporation, State or Country Code	X0
Entity Tax Identification Number	98-1030901
Entity Address, Address Line One	122 LEADENHALL STREET
Entity Address, City or Town	LONDON
Entity Address, Country	GB
Entity Address, Postal Zip Code	EC3V 4AN
City Area Code	20
Local Phone Number	7623 5500
Title of 12(b) Security	Class A Ordinary Shares, $0.01 nominal value
Trading Symbol	AON
Security Exchange Name	NYSE
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
Entity Shell Company	false
Entity Public Float			$ 45,489,377,212
Entity Common Stock, Shares Outstanding		231,582,094
Documents Incorporated by Reference	Aon plc has announced a transaction that, if completed, would result in an Irish public limited company becoming a successor issuer to Aon plc for purposes of Rule 12g-3 under the Securities Exchange Act of 1934, as amended. The proxy statement for the 2020 annual general meeting of shareholders of Aon plc, or if the transaction is completed, of such successor issuer, which meetings are in either case scheduled to be held on June 19, 2020, 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
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY