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Loss and Loss Adjustment Expenses (Details 1) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Summary exposures to various asbestos related claims |  |  |
Reserves for loss and loss adjustment expenses ("LAE")/Reserves for loss and LAE at beginning of year |  |  |
Incurred losses and LAE during the current year | 168 |  |
Less loss and LAE payments for claims occurring during the current year | 1 |  |
Reserves for loss and LAE/Reserves for loss and LAE at end of year | $ 167 |  |

X
- Definition Liability for unpaid claims and claims adjustment expense claims paid current year asbestos. + References No definition available. + Details
Name: | amsf_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYearAsbestos
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Liability for unpaid claims and claims adjustment expense gross asbestos. + References No definition available. + Details
Name: | amsf_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseGrossAsbestos
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Liability for unpaid claims and claims adjustment expense incurred claims asbestos. + References No definition available. + Details
Name: | amsf_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsAsbestos
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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