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Premiums Receivable (Details Textual) (USD $) In Millions, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
--- | --- | --- | ---
Premiums Receivable [Abstract] |  |  |
Earned but Unbilled | $ 3.0 | $ 2.1 | $ 1.8

X
- Definition Earned but unbilled premiums receivable. + References No definition available. + Details
Name: | amsf_EarnedButUnbilledPremiumsReceivable
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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