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Reinsurance - Schedule of Unsecured Reinsurance Recoverables from Reinsurers (Detail) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | $ 85,888 us-gaap_ReinsuranceRecoverables | $ 75,326 us-gaap_ReinsuranceRecoverables
Letters of credit and funds held | (47,833)amsf_FundsHeldAndLettersOfCredit |
Total unsecured reinsurance recoverables | 38,055 amsf_UnsecuredReinsuranceRecoverablesNet |
Ceded Credit Risk, Concentrated Credit Risk [Member] | Hannover Reinsurance (Ireland) Limited (A+) [Member] |  |
Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | 27,387 us-gaap_ReinsuranceRecoverables / us-gaap_CededCreditRiskAxis = amsf_HannoverReinsuranceIrelandLimitedPlusMember / us-gaap_CededCreditRiskRiskClassificationAxis = us-gaap_CededCreditRiskConcentratedCreditRiskMember |
Ceded Credit Risk, Concentrated Credit Risk [Member] | Odyssey America Reinsurance Corporation (A) [Member] |  |
Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | 13,523 us-gaap_ReinsuranceRecoverables / us-gaap_CededCreditRiskAxis = amsf_OdysseyAmericaReinsuranceCorporationMember / us-gaap_CededCreditRiskRiskClassificationAxis = us-gaap_CededCreditRiskConcentratedCreditRiskMember |
Ceded Credit Risk, Concentrated Credit Risk [Member] | Minnesota Workers' Compensation Reinsurance Association (NR) [Member] |  |
Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | 8,518 us-gaap_ReinsuranceRecoverables / us-gaap_CededCreditRiskAxis = amsf_MinnesotaWorkersCompensationReinsuranceAssociationNrMember / us-gaap_CededCreditRiskRiskClassificationAxis = us-gaap_CededCreditRiskConcentratedCreditRiskMember |
Ceded Credit Risk, Concentrated Credit Risk [Member] | Tokio Millenium Reinsurance Limited (A++) [Member] |  |
Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | 6,410 us-gaap_ReinsuranceRecoverables / us-gaap_CededCreditRiskAxis = amsf_TokioMilleniumReinsuranceLimitedAplusplusMember / us-gaap_CededCreditRiskRiskClassificationAxis = us-gaap_CededCreditRiskConcentratedCreditRiskMember |
Ceded Credit Risk, Concentrated Credit Risk [Member] | Clearwater Insurance Company (bbb) [Member] |  |
Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | 6,229 us-gaap_ReinsuranceRecoverables / us-gaap_CededCreditRiskAxis = amsf_ClearwaterInsuranceCompanyBPlusMember / us-gaap_CededCreditRiskRiskClassificationAxis = us-gaap_CededCreditRiskConcentratedCreditRiskMember |
Ceded Credit Risk, Concentrated Credit Risk [Member] | Other Reinsurers [Member] |  |
Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | $ 23,821 us-gaap_ReinsuranceRecoverables / us-gaap_CededCreditRiskAxis = amsf_OtherReinsurersMember / us-gaap_CededCreditRiskRiskClassificationAxis = us-gaap_CededCreditRiskConcentratedCreditRiskMember |

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- Definition Letters of credit from the reinsurer and Amount of the reinsurer's funds held on its behalf by the ceding insurer, including funds held in trust, under reinsurance treaties or contracts with the assuming insurer as security for the payment of the obligations thereunder. + References No definition available. + Details
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Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Unsecured Reinsurance Recoverables Net + References No definition available. + Details
Name: | amsf_UnsecuredReinsuranceRecoverablesNet
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration

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- Definition Amount after valuation allowance recoverable including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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Balance Type: | debit
Period Type: | instant

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- Details
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Balance Type: |
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