Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	May 01, 2021
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Transition Report	false
Entity File Number	1-13653
Entity Registrant Name	AMERICAN FINANCIAL GROUP, INC.
Entity Incorporation, State or Country Code	OH
Entity Tax Identification Number	31-1544320
Entity Central Index Key	0001042046
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
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		85,099,638
Entity Common Stock, Shares Outstanding Owned by Subsidiaries		14,900,000
Common stocks
Entity Information [Line Items]
Title of 12(b) Security	Common Stock
Trading Symbol	AFG
Security Exchange Name	NYSE
5.875% Subordinated Debentures due March 2059
Entity Information [Line Items]
Title of 12(b) Security	5.875% Subordinated Debentures due March 30, 2059
Trading Symbol	AFGB
Security Exchange Name	NYSE
5.625% Subordinated Debentures due June 2060
Entity Information [Line Items]
Title of 12(b) Security	5.625% Subordinated Debentures due June 1, 2060
Trading Symbol	AFGD
Security Exchange Name	NYSE
5.125% Subordinate Debentures due December 2059
Entity Information [Line Items]
Title of 12(b) Security	5.125% Subordinated Debentures due December 15, 2059
Trading Symbol	AFGC
Security Exchange Name	NYSE
4.50% Subordinated Debentures due September 2060
Entity Information [Line Items]
Title of 12(b) Security	4.50% Subordinated Debentures due September 15, 2060
Trading Symbol	AFGE
Security Exchange Name	NYSE