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Fair Value Measurements - Narrative (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) basispoint security professional | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Fair Value Measurements (Textual) [Abstract] |  |  |  |
AFG's internal investment professionals | professional |  | 25 |  |
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 |  | 69.00% |  |
Level 3 assets that were priced using non-binding broker quotes |  | $ 2,410 |  |
Percent of spreads between 400 and 700 basis points |  | 80.00% |  |
Fair value of equity securities transferred into level 3 |  | $ 270 | $ 289 | $ 139
Life contingent annuities |  | 232 | 206 |
Fixed-indexed and variable-indexed annuities (embedded derivative) |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Fair value of derivatives in annuity benefits accumulated measured using a discounted cash flow approach |  | $ 2,720 |  |
Fixed-indexed and variable-indexed annuities (embedded derivative) | Minimum |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Unobservable input surrenders used in Level 3 fair value determination |  | 3.00% |  |
Fixed-indexed and variable-indexed annuities (embedded derivative) | Maximum |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Unobservable input surrenders used in Level 3 fair value determination |  | 23.00% |  |
Fixed-indexed annuities (embedded derivative), majority of future years | Minimum |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Unobservable input surrenders used in Level 3 fair value determination |  | 7.00% |  |
Fixed-indexed annuities (embedded derivative), majority of future years | Maximum |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Unobservable input surrenders used in Level 3 fair value determination |  | 11.00% |  |
Not Designated as Hedging Instrument | Annuity benefits | Fixed-indexed and variable-indexed annuities (embedded derivative) |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Total realized/unrealized gains (losses) included in net income for the embedded derivatives related to the unlocking of actuarial assumptions |  | $ (44) | (25) | (17)
Level 3 |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Internally developed level 3 assets |  | 1,050 |  |
Fixed maturities |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Fair value of equity securities transferred into level 3 |  | 240 | 289 | 124
Equity securities |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Fair value of equity securities transferred into level 3 | $ 29 | 30 | $ 0 | $ 15
Third party pricing model | Fixed maturities | Level 3 |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Internally developed level 3 assets |  | $ 590 |  |
Securities priced using third-party model | security |  | 100 |  |
Basis points credit spread applied by management using third party model | basispoint |  | 585 |  |
Third party pricing model | Fixed maturities | Level 3 | Minimum |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Basis points credit spread applied by management using third party model | basispoint |  | 131 |  |
Third party pricing model | Fixed maturities | Level 3 | Maximum |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Basis points credit spread applied by management using third party model | basispoint |  | 2,966 |  |
Maximum | Third party pricing model | Fixed maturities | Level 3 |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Basis points credit spread applied by management using third party model | basispoint |  | 700 |  |
Minimum | Third party pricing model | Fixed maturities | Level 3 |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Basis points credit spread applied by management using third party model | basispoint |  | 400 |  |

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