| Document and Entity Information | Oct. 29, 2019 |
| Document And Entity Information [Line Items] | |
| Amendment Flag | false |
| Entity Central Index Key | 0000315293 |
| Document Type | 8-K |
| Document Period End Date | Oct. 29, 2019 |
| Entity Registrant Name | Aon plc |
| Entity Incorporation State Country Code | X0 |
| Entity File Number | 1-7933 |
| 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 |
| Written Communications | false |
| Soliciting Material | true |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Class A Ordinary Shares, $0.01 nominal value |
| Trading Symbol | AON |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
