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Investments - Additional Information (Detail) (USD $) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 Individual | Dec. 31, 2013
Investment [Line Items] |  |
Short term debt | $ 33,684,000 us-gaap_ShortTermInvestments | $ 84,422,000 us-gaap_ShortTermInvestments
Carrying value of investments | 11,700,000 amsf_EquityMethodInvestmentsInvestmentCarryingValue |
Held to maturity investments | 19,800,000 us-gaap_AssetsHeldByInsuranceRegulators |
Individual fixed maturity held in unrealized loss position | 174 amsf_NumberOfIndividualFixedMaturitySecuritiesInUnrealizedLossPosition |
Individual fixed maturity held in unrealized loss position | 19 amsf_NumberOfIndividualInvestmentSecuritiesInUnrealizedLossPositionGreaterThanTwelveMonths |
Impairment losses recognized | 222,000 us-gaap_ImpairmentOfInvestments |
Net carrying value of security held | 2,100,000 amsf_HeldToMaturitySecuritiesPutSecurityAtCarryingValue |
Sale of security loss realized | 58,000 amsf_HeldToMaturitySecuritiesPutSecurityRealizedGainLossExcludingOtherThanTemporaryImpairments |
Fixed Maturity Securities Available for Sale [Member] |  |
Investment [Line Items] |  |
Impairment losses recognized | 222,000 us-gaap_ImpairmentOfInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember |
Corporate Bonds [Member] |  |
Investment [Line Items] |  |
Short term debt | 32,700,000 us-gaap_ShortTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_CorporateBondSecuritiesMember | 82,700,000 us-gaap_ShortTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_CorporateBondSecuritiesMember
Certificates of Deposit [Member] |  |
Investment [Line Items] |  |
Short term debt | 1,000,000 us-gaap_ShortTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_CertificatesOfDepositMember | 900,000 us-gaap_ShortTermInvestments / us-gaap_InvestmentTypeAxis = us-gaap_CertificatesOfDepositMember
Municipal Securities [Member] |  |
Investment [Line Items] |  |
Short term debt |  | $ 800,000 us-gaap_ShortTermInvestments / us-gaap_InvestmentTypeAxis = amsf_MunicipalSecuritiesMember

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- Definition Equity method investments investment carrying value. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition For any redemption o.f of securities categorized as held to maturity, this item represents the net carrying amount of the redeemed security. + References No definition available. + Details
Name: | amsf_HeldToMaturitySecuritiesPutSecurityAtCarryingValue
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition This item represents the gain (loss) realized on the sale of held to maturity securities disposed of during the period. + References No definition available. + Details
Name: | amsf_HeldToMaturitySecuritiesPutSecurityRealizedGainLossExcludingOtherThanTemporaryImpairments
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Number of individual fixed maturity securities in unrealized loss position. + References No definition available. + Details
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Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Number of individual investment securities in unrealized loss position greater than twelve months. + References No definition available. + Details
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- Details
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- Definition Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 320 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28366721&loc=d3e22002-158495 + Details
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Period Type: | instant

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- Definition The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25351-111560 + Details
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- Definition Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph g -Article 7 + Details
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- Details
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- Details
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- Details
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- Details
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