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Deferred Policy Acquisition Costs - Narrative (Details) - USD ($) $ in Millions | Jun. 30, 2020 | Dec. 31, 2019
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Deferred Policy Acquisition Costs Disclosures [Abstract] |  |
Accumulated amortization of present value of future profits | $ 157 | $ 154

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- Definition The cumulative amount of amortization of present value of future profits that has been recognized in the Income Statement. + References No definition available. + Details
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Namespace Prefix: | afg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
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