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Commitments and Contingencies (Details Textual) (USD $) | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Commitments and Contingencies (Textual) [Abstract] |  |  |
Rent expense | $ 237,000 | $ 293,000 | $ 475,000

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- Details
Name: | amsf_CommitmentsAndContingenciesTextualAbstract
Namespace Prefix: | amsf_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Rental expense incurred for leased assets including furniture and equipment which has not been recognized in costs and expenses applicable to sales and revenues; for example, cost of goods sold or other operating costs and expenses. + References No definition available. + Details
Name: | us-gaap_LeaseAndRentalExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration