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Reinsurance - Schedule of Unsecured Reinsurance Recoverables from Reinsurers (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Ceded Credit Risk [Line Items]    
Total reinsurance recoverables $ 85,888us-gaap_ReinsuranceRecoverables $ 75,326us-gaap_ReinsuranceRecoverables
Letters of credit and funds held (47,833)amsf_FundsHeldAndLettersOfCredit  
Total unsecured reinsurance recoverables 38,055amsf_UnsecuredReinsuranceRecoverablesNet  
Ceded Credit Risk, Concentrated Credit Risk [Member] | Hannover Reinsurance (Ireland) Limited (A+) [Member]    
Ceded Credit Risk [Line Items]    
Total reinsurance recoverables 27,387us-gaap_ReinsuranceRecoverables
/ us-gaap_CededCreditRiskAxis
= amsf_HannoverReinsuranceIrelandLimitedPlusMember
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember		  
Ceded Credit Risk, Concentrated Credit Risk [Member] | Odyssey America Reinsurance Corporation (A) [Member]    
Ceded Credit Risk [Line Items]    
Total reinsurance recoverables 13,523us-gaap_ReinsuranceRecoverables
/ us-gaap_CededCreditRiskAxis
= amsf_OdysseyAmericaReinsuranceCorporationMember
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember		  
Ceded Credit Risk, Concentrated Credit Risk [Member] | Minnesota Workers' Compensation Reinsurance Association (NR) [Member]    
Ceded Credit Risk [Line Items]    
Total reinsurance recoverables 8,518us-gaap_ReinsuranceRecoverables
/ us-gaap_CededCreditRiskAxis
= amsf_MinnesotaWorkersCompensationReinsuranceAssociationNrMember
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember		  
Ceded Credit Risk, Concentrated Credit Risk [Member] | Tokio Millenium Reinsurance Limited (A++) [Member]    
Ceded Credit Risk [Line Items]    
Total reinsurance recoverables 6,410us-gaap_ReinsuranceRecoverables
/ us-gaap_CededCreditRiskAxis
= amsf_TokioMilleniumReinsuranceLimitedAplusplusMember
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember		  
Ceded Credit Risk, Concentrated Credit Risk [Member] | Clearwater Insurance Company (bbb) [Member]    
Ceded Credit Risk [Line Items]    
Total reinsurance recoverables 6,229us-gaap_ReinsuranceRecoverables
/ us-gaap_CededCreditRiskAxis
= amsf_ClearwaterInsuranceCompanyBPlusMember
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember		  
Ceded Credit Risk, Concentrated Credit Risk [Member] | Other Reinsurers [Member]    
Ceded Credit Risk [Line Items]    
Total reinsurance recoverables $ 23,821us-gaap_ReinsuranceRecoverables
/ us-gaap_CededCreditRiskAxis
= amsf_OtherReinsurersMember
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember