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Reinsurance - Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Insurance [Abstract] |  |  |
Paid losses recoverable outstanding due date | 90 days |  |
Paid losses recoverable past due | $ 0 amsf_PaidLossRecoverablePastDue |  |
Return of previous recoveries |  | 2,300,000 amsf_ReturnOfPriorReinsuranceRecoveries |
Received reinsurance recoveries | $ 1,500,000 amsf_ProceedsFromReinsuranceRecoveries | $ 2,100,000 amsf_ProceedsFromReinsuranceRecoveries | $ 5,300,000 amsf_ProceedsFromReinsuranceRecoveries
Unsecured reinsurance recoverable by reinsurer reporting threshold | 1.50%amsf_PercentageLimitForReportingUnsecuredReinsuranceRecoverables |  |

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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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- Definition Return Of Prior Reinsurance Recoveries + References No definition available. + Details
Name: | amsf_ReturnOfPriorReinsuranceRecoveries
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
Name: | us-gaap_InsuranceAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration