Fair Value Measurements - Narrative (Details) $ in Millions	Sep. 30, 2026 USD ($) professional
Fair Value Disclosures [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 internally developed level 3 assets compared to total level 3 assets	88.00%
Internally developed level 3 assets	$ 877
Percentage of level 3 assets that were priced using a discounted cash flow approach	73.00%
Level 3 assets that were priced using a discounted cash flow approach	$ 644
Level 3 assets that were priced using unobservable internal models and third party inputs	$ 233
Percentage of level 3 assets that were priced using financial information from third parties	8.00%
Level 3 assets that were priced using financial information from third parties	$ 78
Percentage of level 3 assets that were priced using non-binding broker quotes or pricing services	4.00%
Level 3 assets that were priced using non-binding broker quotes or pricing services	$ 42

Level 3 assets in which the fair value was developed internally.

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Level three assets as a percentage of assets measured at fair value

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Level three assets that were priced using a discounted cash flow approach.

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Level three assets that were priced using financial information from third parties

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Level three assets that were priced using non-binding broker quotes or pricing services.

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No definition available.

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Level three assets that were priced using unobservable internal models and third party inputs

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No definition available.

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Number of internal investment professionals.

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Percentage Of Internally Developed Level Three Assets Compared to Total Level 3 Assets

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Percentage of level three assets that were priced using a discounted cash flow approach.

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Percentage of level three assets that were priced using financial information from third parties

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Percentage of level 3 assets that were priced using non-binding broker quotes or pricing services.

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