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Reinsurance - Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Insurance [Abstract] |  |  |
Paid losses recoverable outstanding due date | 90 days |  |
Return of previous recoveries | $ 2,316,000 |  |
Received reinsurance recoveries | $ 2,078,000 | $ 5,344,000 | $ 2,861,000
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
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- Definition Threshold For Reporting Unsecured Reinsurance Recoverables. + References No definition available. + Details
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Namespace Prefix: | amsf_
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- Definition Proceeds From Reinsurance Recoveries + References No definition available. + Details
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Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Return Of Prior Reinsurance Recoveries + References No definition available. + Details
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Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Details
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