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Reinsurance - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Insurance [Abstract]      
Paid losses recoverable outstanding due date 90 days    
Paid losses recoverable past due $ 0amsf_PaidLossRecoverablePastDue    
Return of previous recoveries   2,300,000amsf_ReturnOfPriorReinsuranceRecoveries  
Received reinsurance recoveries $ 1,500,000amsf_ProceedsFromReinsuranceRecoveries $ 2,100,000amsf_ProceedsFromReinsuranceRecoveries $ 5,300,000amsf_ProceedsFromReinsuranceRecoveries
Unsecured reinsurance recoverable by reinsurer reporting threshold 1.50%amsf_PercentageLimitForReportingUnsecuredReinsuranceRecoverables