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Fair Value Measurements - Schedule of Assets and Liabilities Measured at Amortized Cost (Detail) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | $ 664,371 us-gaap_HeldToMaturitySecuritiesFairValue | $ 561,179 us-gaap_HeldToMaturitySecuritiesFairValue
States and Political Subdivisions [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 405,665 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 399,155 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Corporate Bonds [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 176,734 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CorporateDebtSecuritiesMember | 68,243 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CorporateDebtSecuritiesMember
Commercial Mortgage-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 48,529 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 54,244 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 18,672 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember | 23,565 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
US Treasury Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 11,792 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_USTreasurySecuritiesMember | 6,602 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_USTreasurySecuritiesMember
Asset-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 2,979 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_AssetBackedSecuritiesMember | 3,456 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_AssetBackedSecuritiesMember
Obligations of U.S. Government Agencies [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value |  | 5,914 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = amsf_ObligationsOfUsGovernmentAgenciesMember
Fair Value, Inputs, Level 1 [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 11,792 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 6,602 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 11,792 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_USTreasurySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 6,602 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_USTreasurySecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 652,579 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 554,577 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fair Value, Inputs, Level 2 [Member] | States and Political Subdivisions [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 405,665 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 399,155 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fair Value, Inputs, Level 2 [Member] | Corporate Bonds [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 176,734 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 68,243 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fair Value, Inputs, Level 2 [Member] | Commercial Mortgage-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 48,529 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 54,244 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fair Value, Inputs, Level 2 [Member] | U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 18,672 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 23,565 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fair Value, Inputs, Level 2 [Member] | Asset-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 2,979 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 3,456 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fair Value, Inputs, Level 2 [Member] | Obligations of U.S. Government Agencies [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value |  | $ 5,914 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = amsf_ObligationsOfUsGovernmentAgenciesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member

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- Definition Fair value of investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 + Details
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