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Loss and Loss Adjustment Expenses - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Unpaid Losses And Loss Expenses [Line Items]      
Loss and LAE Related to Prior Periods $ (23,717)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense $ (12,611)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense $ (2,490)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense
Accident Year 2012 [Member]      
Unpaid Losses And Loss Expenses [Line Items]      
Loss and LAE Related to Prior Periods (9,300)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense
/ amsf_LossContingencyByPeriodAxis
= amsf_AccidentYear2012Member		    
Accident Year 2010 [Member]      
Unpaid Losses And Loss Expenses [Line Items]      
Loss and LAE Related to Prior Periods (6,900)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense
/ amsf_LossContingencyByPeriodAxis
= amsf_AccidentYear2010Member		    
Accident Year 2009 [Member]      
Unpaid Losses And Loss Expenses [Line Items]      
Loss and LAE Related to Prior Periods $ (1,700)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense
/ amsf_LossContingencyByPeriodAxis
= amsf_AccidentYear2009Member