Fair Value Measurements - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) basispoint professional security	Dec. 31, 2019 USD ($)
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	43.00%
Level 3 assets that were priced using non-binding broker quotes | $	$ 1,520
Percent of spreads between 200 and 700 basis points	70.00%
Life contingent annuities | $	$ 245	$ 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	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 | $	$ 3,100
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	4.00%
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	21.00%
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Internally developed level 3 assets | $	$ 1,630
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 | $	$ 727
Securities priced using a third-party model | security	37
Basis points credit spread applied by management using third party model | basispoint	414
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
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
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	53
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