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Comprehensive Income and Accumulated Other Comprehensive Income (Loss) - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2013
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract] |
Comprehensive income tax rate | 35.00%

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- Definition Other Comprehensive Income Loss Available For Sale Securities Tax Rate + References No definition available. + Details
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Namespace Prefix: | amsf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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