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Reinsurance - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Insurance [Abstract] |  |  |
Paid losses recoverable outstanding due date | 90 days |  |
Paid losses recoverable past due | $ 0 |  |
Received reinsurance recoveries | $ 1,300,000 | $ 1,600,000 | $ 500,000
Unsecured reinsurance recoverable by reinsurer reporting threshold | 1.50% |  |

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- Definition Amount after valuation allowance currently billed or billable under reinsurance contracts which are more than 90 days and considered to be past due. + References No definition available. + Details
Name: | amsf_PaidLossRecoverablePastDue
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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: | num: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