| Document and Entity Information | Apr. 30, 2020 |
| Entity Listings [Line Items] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Apr. 30, 2020 |
| Entity Registrant Name | AMERICAN FINANCIAL GROUP, INC. |
| Entity Incorporation, State or Country Code | OH |
| Entity File Number | 001-13653 |
| Entity Tax Identification Number | 31-1544320 |
| Entity Address, Address Line One | 301 East Fourth Street |
| Entity Address, City or Town | Cincinnati |
| Entity Address, State or Province | OH |
| Entity Address, Postal Zip Code | 45202 |
| City Area Code | 513 |
| Local Phone Number | 579-2121 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001042046 |
| Common Stock [Member] | |
| Entity Listings [Line Items] | |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | AFG |
| Security Exchange Name | NYSE |
| 6% Subordinated Debentures due November 15, 2055 [Member] | |
| Entity Listings [Line Items] | |
| Title of 12(b) Security | 6% Subordinated Debentures due November 15, 2055 |
| Trading Symbol | AFGH |
| Security Exchange Name | NYSE |
| 5.875% Subordinated Debentures due March 30, 2059 [Member] | |
| Entity Listings [Line Items] | |
| Title of 12(b) Security | 5.875% Subordinated Debentures due March 30, 2059 |
| Trading Symbol | AFGB |
| Security Exchange Name | NYSE |
| 5.125% Subordinated Debentures due December 15, 2059 [Member] | |
| Entity Listings [Line Items] | |
| Title of 12(b) Security | 5.125% Subordinated Debentures due December 15, 2059 |
| Trading Symbol | AFGC |
| Security Exchange Name | NYSE |
