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Comprehensive Income and Accumulated Other Comprehensive Income (Loss) - Other Comprehensive Income (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Equity [Abstract]      
Unrealized gain (loss) on available-for-sale securities, Pre-tax Amount $ 10,133us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax $ (11,504)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax $ 2,555us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax
Change in unrealized losses on fixed maturity securities with OTTI, Pre-Tax Amount 17amsf_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForHeldToMaturitySecuritiesBeforeTax 2amsf_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForHeldToMaturitySecuritiesBeforeTax 1amsf_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForHeldToMaturitySecuritiesBeforeTax
Change in unrealized losses on equity securities with OTTI, Pre-Tax Amount   416amsf_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsBeforeTax  
Less amortization of differences between fair value and amortized cost for fixed maturity security transfer, Pre-tax Amount (44)amsf_OtherComprehensiveIncomeReclassificationAdjustmentForAvailableForSaleTransferredToHeldToMaturitySecuritiesBeforeTax (75)amsf_OtherComprehensiveIncomeReclassificationAdjustmentForAvailableForSaleTransferredToHeldToMaturitySecuritiesBeforeTax (101)amsf_OtherComprehensiveIncomeReclassificationAdjustmentForAvailableForSaleTransferredToHeldToMaturitySecuritiesBeforeTax
Less reclassification adjustment for gains realized in net income, Pre-Tax Amount 825us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax (30)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax (1,279)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
Net unrealized gain loss, Pre-Tax Amount 10,931amsf_UnrealizedGainLossOnSecuritiesBeforeTax (11,191)amsf_UnrealizedGainLossOnSecuritiesBeforeTax 1,176amsf_UnrealizedGainLossOnSecuritiesBeforeTax
Other comprehensive income, Pre-Tax Amount 10,931us-gaap_OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParent (11,191)us-gaap_OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParent 1,176us-gaap_OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParent
Unrealized gain loss on available-for-sale securities, Tax Expense 3,546us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax (4,027)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax 894us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax
Change in unrealized losses on fixed maturity securities with OTTI, Tax Expense 6amsf_OtherThanTemporaryImpairmentGainsLossesInvestmentsPortionInOtherComprehensiveLossTaxPortionAttributableToParent 1amsf_OtherThanTemporaryImpairmentGainsLossesInvestmentsPortionInOtherComprehensiveLossTaxPortionAttributableToParent  
Change in unrealized losses on equity securities with OTTI, Tax Expense   146us-gaap_OtherComprehensiveIncomeAvailableforsaleSecuritiesTaxPortionAttributableToParent  
Less amortization of differences between fair value and amortized cost for fixed maturity security transfer, Tax Expense (15)amsf_OtherComprehensiveIncomeReclassificationAdjustmentForAvailableForSaleTransferredToHeldToMaturitySecuritiesTax (26)amsf_OtherComprehensiveIncomeReclassificationAdjustmentForAvailableForSaleTransferredToHeldToMaturitySecuritiesTax (35)amsf_OtherComprehensiveIncomeReclassificationAdjustmentForAvailableForSaleTransferredToHeldToMaturitySecuritiesTax
Less reclassification adjustment for gains realized in net income, Tax Expense 289us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax (11)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax (447)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax
Net unrealized gain loss, Tax Expense 3,826amsf_UnrealizedGainLossOnSecuritiesTax (3,917)amsf_UnrealizedGainLossOnSecuritiesTax 412amsf_UnrealizedGainLossOnSecuritiesTax
Other comprehensive income, Tax Expense 3,826us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1 (3,917)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1 412us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
Unrealized gain (loss) on available-for-sale securities, Net-of-Tax Amount 6,587amsf_OtherComprehensiveIncomeUnrealizedGainLossOnAvailableForSaleSecuritiesArisingDuringPeriodNetOfTax (7,477)amsf_OtherComprehensiveIncomeUnrealizedGainLossOnAvailableForSaleSecuritiesArisingDuringPeriodNetOfTax 1,661amsf_OtherComprehensiveIncomeUnrealizedGainLossOnAvailableForSaleSecuritiesArisingDuringPeriodNetOfTax
Change in unrealized losses on fixed maturity securities with OTTI, Net-of-Tax Amount 11amsf_OtherThanTemporaryImpairmentGainsLossesInvestmentsPortionInOtherComprehensiveLossNetOftaxPortionAttributableToParent 1amsf_OtherThanTemporaryImpairmentGainsLossesInvestmentsPortionInOtherComprehensiveLossNetOftaxPortionAttributableToParent 1amsf_OtherThanTemporaryImpairmentGainsLossesInvestmentsPortionInOtherComprehensiveLossNetOftaxPortionAttributableToParent
Change in unrealized losses on equity securities with OTTI, Net-of-Tax Amount   270us-gaap_OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent  
Less amortization of differences between fair value and amortized cost for fixed maturity security transfer, Net-of-Tax Amount (29)amsf_OtherComprehensiveIncomeReclassificationAdjustmentForAvailableForSaleTransferredToHeldToMaturitySecuritiesNetOfTax (49)amsf_OtherComprehensiveIncomeReclassificationAdjustmentForAvailableForSaleTransferredToHeldToMaturitySecuritiesNetOfTax (66)amsf_OtherComprehensiveIncomeReclassificationAdjustmentForAvailableForSaleTransferredToHeldToMaturitySecuritiesNetOfTax
Reclassification adjustment for gains realized in net income, Net-of-Tax Amount 536us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax (19)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax (832)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax
Net unrealized gain loss, Net-of-Tax Amount 7,105amsf_UnrealizedGainLossOnSecuritiesNetOfTax (7,274)amsf_UnrealizedGainLossOnSecuritiesNetOfTax 764amsf_UnrealizedGainLossOnSecuritiesNetOfTax
Other comprehensive income, Net-of-Tax Amount $ 7,105us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent $ (7,274)us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent $ 764us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent