Fair Value Measurements - Narrative (Details) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2019 USD ($) basispoint stock security	Jun. 30, 2018 USD ($) stock	Mar. 31, 2018 USD ($)	Jun. 30, 2019 USD ($) basispoint stock professional security	Jun. 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				6.00%
Percentage of level 3 assets that were priced using non-binding broker quotes				55.00%
Level 3 assets that were priced using non-binding broker quotes				$ 1,710
Percent of spreads between 400 and 700 basis points	80.00%			80.00%
Fair value of equity securities transferred into Level 3	$ 41	$ 51		$ 57	$ 87
Total realized/unrealized gains (losses) included in net income for the embedded derivatives related to unlocking of actuarial assumptions					$ 44
Life contingent annuities	238			$ 238		$ 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				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	3,540			$ 3,540
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				23.00%
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Internally developed level 3 assets	$ 1,190			$ 1,190
Perpetual preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of stocks transferred from Level 1 to Level 2 | stock	1	2		1	2
Fair Value of assets transferred Level 1 to Level 2, during the first six months	$ 6	$ 6		$ 6	$ 6
Fair Value of assets transferred Level 1 to Level 2, during the second quarter	6	6		6	6
Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of equity securities transferred into Level 3	35	50		35	57
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of equity securities transferred into Level 3	6	1	$ 29	22	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	$ 833			$ 833
Securities priced using a third-party model | security	140			140
Basis points credit spread applied by management using third party model | basispoint	576			576
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) [Member]
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		$ 44			$ 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