XML 104 R80.htm IDEA: XBRL DOCUMENT v2.4.1.9
Stock Options and Restricted Stock - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total tax expense $ 2,841,000us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities $ 2,024,000us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities $ 421,000us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities  
2005 Incentive Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock options exercise price 100.00%amsf_ExercisePriceAsPercentageOfFairMarketValueOfUnderlyingSharesOnDateOfGrant
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
     
Company's voting stock 10.00%amsf_ShareBasedCompensationArrangementGrantRestrictionVotingPercentage
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
     
Stock options exercise price 110.00%amsf_ExercisePriceAsPercentageOfFairMarketValueOfUnderlyingSharesOnDateOfGrantOne
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
     
Duration for Stock option grants exercisable 10 years      
Stock options exercise period 5 years      
Expiration of stock option 90 days      
Unexercised portion of stock option remains exercisable 1 year      
Number of companies used for the estimation of expected volatility 6amsf_NumberOfCompaniesUsedForEstimationOfExpectedVolatility
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
     
Dividend yield assumed 0.00%us-gaap_FairValueAssumptionsExpectedDividendRate
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
     
Cash dividends paid   0us-gaap_DividendsShareBasedCompensationCash
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
   
Compensation expense 300,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
300,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
200,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
 
Total tax expense 38,000us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
4,000us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
   
Restricted stock outstanding 2,400us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
6,400us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
8,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
 
Restricted stock granted     8,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_PlanNameAxis
= amsf_TwoThousandFiveIncentivePlanMember
 
2010 Restricted Stock Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Compensation expense 200,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
200,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
200,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
 
Total tax expense 7,000us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
18,000us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
16,000us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
 
Shares authorized under incentive plan 100,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
     
Common stock available for future awards 33,186us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
     
Restricted stock award for a number of shares 30,000amsf_RestrictedStockAwardForNumberOfStocks
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
     
Restricted stock outstanding 5,229us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
5,376us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
7,833us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
9,513us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
Restricted stock granted 5,229us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
5,376us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
7,833us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTenRestrictedStockPlanMember
 
2012 Equity and Incentive Compensation Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares authorized under incentive plan 500,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember
     
Common stock available for future awards 413,079us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember
     
Restricted stock granted 4,312us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember
     
2012 Equity and Incentive Compensation Plan [Member] | Share Based Grants [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Compensation expense 600,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= amsf_ShareBasedGrantsMember
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember
300,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= amsf_ShareBasedGrantsMember
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember
12,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= amsf_ShareBasedGrantsMember
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember
 
2012 Equity and Incentive Compensation Plan [Member] | Long-Term Incentive Awards [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Compensation expense $ 400,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= amsf_LongTermIncentiveAwardsMember
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember
$ 600,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= amsf_LongTermIncentiveAwardsMember
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember
$ 200,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= amsf_LongTermIncentiveAwardsMember
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember
 
2012 Equity and Incentive Compensation Plan [Member] | Restricted Stock Units (RSUs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted stock granted   80,278us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember