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Reinsurance - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Insurance [Abstract] |  |  |
Paid losses recoverable outstanding due date | 90 days |  |
Received reinsurance recoveries | $ 0.3 | $ 16.0 | $ 1.4
Unsecured reinsurance recoverable by reinsurer reporting threshold | 1.50% |  |

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- Definition Paid Loss Recoverable Past Due Period + References No definition available. + Details
Name: | amsf_PaidLossRecoverablePastDuePeriod
Namespace Prefix: | amsf_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Threshold For Reporting Unsecured Reinsurance Recoverables. + References No definition available. + Details
Name: | amsf_PercentageLimitForReportingUnsecuredReinsuranceRecoverables
Namespace Prefix: | amsf_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Proceeds From Reinsurance Recoveries + References No definition available. + Details
Name: | amsf_ProceedsFromReinsuranceRecoveries
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_InsuranceAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration