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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Schedule Of Accounting Policies [Line Items]      
Percentage of comparable consolidated amounts 95.00%amsf_PercentageOfComparableConsolidatedAmount 95.00%amsf_PercentageOfComparableConsolidatedAmount 95.00%amsf_PercentageOfComparableConsolidatedAmount
Original maturity of cash equivalent 3 months    
Experience rated commission and other operating costs $ 0amsf_ExperienceRelatedCommission $ 9,300,000amsf_ExperienceRelatedCommission $ 11,100,000amsf_ExperienceRelatedCommission
Advertising expense 500,000us-gaap_AdvertisingExpense 500,000us-gaap_AdvertisingExpense 400,000us-gaap_AdvertisingExpense
Premium tax benefit 4,000,000amsf_PremiumTaxBenefitAccrued 3,600,000amsf_PremiumTaxBenefitAccrued  
Assessment Related premium Payment period 1 year    
Assessment Related loss Payment period 1 year    
Land and Office Building [Member]      
Schedule Of Accounting Policies [Line Items]      
Estimated useful life of assets 39 years    
Limited Partnership [Member]      
Schedule Of Accounting Policies [Line Items]      
Unfunded commitments $ 0amsf_UnfundedCommitments
/ us-gaap_PartnerTypeOfPartnersCapitalAccountAxis
= us-gaap_LimitedPartnerMember		    
Maximum [Member]      
Schedule Of Accounting Policies [Line Items]      
Short-term investments maturity period P1Y    
Maximum [Member] | Other Fixed Assets [Member]      
Schedule Of Accounting Policies [Line Items]      
Estimated useful life of assets 7 years    
Minimum [Member]      
Schedule Of Accounting Policies [Line Items]      
Short-term investments maturity period P3M    
Minimum [Member] | Other Fixed Assets [Member]      
Schedule Of Accounting Policies [Line Items]      
Estimated useful life of assets 3 years