Fair Value Measurements - Narrative (Details) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2019 USD ($) basispoint security stock	Sep. 30, 2018 USD ($) stock	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2019 USD ($) basispoint security professional stock	Sep. 30, 2018 USD ($) stock	Dec. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
AFG's internal investment professionals | professional					20
Level 3 assets as a percentage of total assets measured at fair value					7.00%
Percentage of level 3 assets that were priced using non-binding broker quotes					49.00%
Level 3 assets that were priced using non-binding broker quotes					$ 1,720
Percent of spreads between 400 and 700 basis points	80.00%				80.00%
Fair value of equity securities transferred into Level 3	$ 88	$ 13			$ 145	$ 100
Total realized/unrealized gains (losses) included in net income for the embedded derivatives related to unlocking of actuarial assumptions	181		$ (44)
Life contingent annuities	250				$ 250		$ 232
Fixed-indexed and variable-indexed annuities (embedded derivative), majority of future years | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input surrenders used in Level 3 fair value determination					7.00%
Fixed-indexed and variable-indexed annuities (embedded derivative), majority of future years | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input surrenders used in Level 3 fair value determination					10.00%
Fixed-indexed and variable-indexed annuities (embedded derivative)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivatives in annuity benefits accumulated measured using a discounted cash flow approach	3,470				$ 3,470
Fixed-indexed and variable-indexed annuities (embedded derivative) | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input surrenders used in Level 3 fair value determination					3.00%
Fixed-indexed and variable-indexed annuities (embedded derivative) | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input surrenders used in Level 3 fair value determination					22.00%
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Internally developed level 3 assets	$ 1,480				$ 1,480
Perpetual preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of stocks that transferred from Level 2 to Level 1, during the period | stock	2
Number of stocks transferred from Level 2 to Level 1 | stock					2
Fair Value of assets transferred Level 2 to Level 1, during the period	$ 11
Fair Value of assets transferred Level 2 to Level 1	11				$ 11
Number of stocks transferred from Level 1 to Level 2 | stock					1	2
Fair Value of assets transferred Level 1 to Level 2	6	$ 6			$ 6	$ 6
Number of stocks transferred from Level 1 to Level 2, during the period | stock		0
Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of equity securities transferred into Level 3	54	$ 13			89	70
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of equity securities transferred into Level 3	34	$ 0		$ 29	56	30
Third party pricing model | Fixed maturities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Internally developed level 3 assets	$ 998				$ 998
Securities priced using a third-party model | security	175				175
Basis points credit spread applied by management using third party model | basispoint	568				568
Third party pricing model | Fixed maturities | Level 3 | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Basis points credit spread applied by management using third party model | basispoint	400				400
Third party pricing model | Fixed maturities | Level 3 | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Basis points credit spread applied by management using third party model | basispoint	700				700
Not Designated as Hedging Instrument | Annuity benefits | Fixed-Indexed Annuities (Embedded Derivative)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total realized/unrealized gains (losses) included in net income for the embedded derivatives related to unlocking of actuarial assumptions	$ 181				$ 181	$ (44)
Credit spread used in pricing model | Third party pricing model | Fixed maturities | Level 3 | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Basis points credit spread applied by management using third party model | basispoint	100				100
Credit spread used in pricing model | Third party pricing model | Fixed maturities | Level 3 | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Basis points credit spread applied by management using third party model | basispoint	2,966				2,966