Fair Value Measurements - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) basispoint security	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) basispoint security professional	Sep. 30, 2019 USD ($)	Dec. 31, 2019 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			38.00%
Level 3 assets that were priced using non-binding broker quotes			$ 1,460
Percent of spreads between 200 and 700 basis points	80.00%		80.00%
Life contingent annuities	$ 850		$ 850		$ 247
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			8.00%
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			11.00%
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	3,660		$ 3,660
Embedded derivatives | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input surrenders used in Level 3 fair value determination			4.00%
Embedded derivatives | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input surrenders used in Level 3 fair value determination			23.00%
Fixed-Indexed Annuities (Embedded Derivative) | Not designated as hedging instrument | Annuity benefits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total realized/unrealized gains (losses) related to unlocking of actuarial assumptions	240	$ 181	$ 240	$ 181
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Internally developed level 3 assets	1,740		1,740
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	$ 730		$ 730
Securities priced using a third-party model | security	43		43
Basis points credit spread applied by management using third party model | basispoint	397		397
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	200		200
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
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	1,253		1,253