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Fair Value Measurements - Narrative (Details) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 USD ($) stock | Mar. 31, 2018 USD ($) | Jun. 30, 2017 USD ($) stock | Jun. 30, 2018 USD ($) stock professional | Jun. 30, 2017 USD ($) stock | Dec. 31, 2017 USD ($)
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 |  |  |  | 6.00% |  |
Percentage of level 3 assets that were priced using non-binding broker quotes |  |  |  | 73.00% |  |
Level 3 assets that were priced using non-binding broker quotes |  |  |  | $ 2,140 |  |
Fair value of equity securities transferred into Level 3 | $ 51 |  | $ 32 | 87 | $ 89 |
Life contingent annuities | 213 |  |  | $ 213 |  | $ 206
Maximum |  |  |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |  |  |
Percentage of internally developed Level 3 asset fair values to Shareholders' equity (are less than) |  |  |  | 14.00% |  |
Fixed-indexed and variable-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 and variable-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% |  |
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 | 2,780 |  |  | $ 2,780 |  |
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% |  |
Equity securities |  |  |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |  |  |
Fair value of equity securities transferred into Level 3 | $ 1 | $ 29 | $ 0 | $ 30 | $ 0 |
Preferred stocks |  |  |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |  |  |
Number of stocks transferred from Level 1 to Level 2 | stock | 2 |  |  | 2 |  |
Fair Value of assets transferred from Level 1 to Level 2, during the second quarter | $ 6 |  |  |  |  |
Fair Value of assets transferred from Level 1 to Level 2, during the first six months | $ 6 |  |  | $ 6 |  |
Number of stocks transferred from Level 2 to Level 1 | stock |  |  | 2 |  | 2 |
Fair Value of assets transferred from Level 2 to Level 1, during the second quarter |  |  | $ 16 |  |  |
Fair Value of assets transferred from Level 2 to Level 1, during the first six months |  |  | $ 16 |  | $ 16 |

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