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Loss and Loss Adjustment Expenses - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Unpaid Losses And Loss Expenses [Line Items] |  |  |
Loss and LAE Related to Prior Periods | $ (23,717)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense | $ (12,611)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense | $ (2,490)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense
Accident Year 2012 [Member] |  |  |
Unpaid Losses And Loss Expenses [Line Items] |  |  |
Loss and LAE Related to Prior Periods | (9,300)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense / amsf_LossContingencyByPeriodAxis = amsf_AccidentYear2012Member |  |
Accident Year 2010 [Member] |  |  |
Unpaid Losses And Loss Expenses [Line Items] |  |  |
Loss and LAE Related to Prior Periods | (6,900)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense / amsf_LossContingencyByPeriodAxis = amsf_AccidentYear2010Member |  |
Accident Year 2009 [Member] |  |  |
Unpaid Losses And Loss Expenses [Line Items] |  |  |
Loss and LAE Related to Prior Periods | $ (1,700)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense / amsf_LossContingencyByPeriodAxis = amsf_AccidentYear2009Member |  |

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- Definition Amount of expense for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 + Details
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