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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Schedule Of Accounting Policies [Line Items] |  |  |
Percentage of comparable consolidated amounts | 95.00%amsf_PercentageOfComparableConsolidatedAmount | 95.00%amsf_PercentageOfComparableConsolidatedAmount | 95.00%amsf_PercentageOfComparableConsolidatedAmount
Original maturity of cash equivalent | 3 months |  |
Experience rated commission and other operating costs | $ 0 amsf_ExperienceRelatedCommission | $ 9,300,000 amsf_ExperienceRelatedCommission | $ 11,100,000 amsf_ExperienceRelatedCommission
Advertising expense | 500,000 us-gaap_AdvertisingExpense | 500,000 us-gaap_AdvertisingExpense | 400,000 us-gaap_AdvertisingExpense
Premium tax benefit | 4,000,000 amsf_PremiumTaxBenefitAccrued | 3,600,000 amsf_PremiumTaxBenefitAccrued |
Assessment Related premium Payment period | 1 year |  |
Assessment Related loss Payment period | 1 year |  |
Land and Office Building [Member] |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |
Estimated useful life of assets | 39 years |  |
Limited Partnership [Member] |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |
Unfunded commitments | $ 0 amsf_UnfundedCommitments / us-gaap_PartnerTypeOfPartnersCapitalAccountAxis = us-gaap_LimitedPartnerMember |  |
Maximum [Member] |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |
Short-term investments maturity period | P1Y |  |
Maximum [Member] | Other Fixed Assets [Member] |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |
Estimated useful life of assets | 7 years |  |
Minimum [Member] |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |
Short-term investments maturity period | P3M |  |
Minimum [Member] | Other Fixed Assets [Member] |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |
Estimated useful life of assets | 3 years |  |

X
- Definition Assessments Related Losses Payment Period Minimum + References No definition available. + Details
Name: | amsf_AssessmentsRelatedLossesPaymentPeriodMinimum
Namespace Prefix: | amsf_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Assessments Related Premiums Payment Period Minimum + References No definition available. + Details
Name: | amsf_AssessmentsRelatedPremiumsPaymentPeriodMinimum
Namespace Prefix: | amsf_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Cash and cash equivalent maturity period. + References No definition available. + Details
Name: | amsf_CashAndCashEquivalentMaturityPeriod
Namespace Prefix: | amsf_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Experience related commission. + References No definition available. + Details
Name: | amsf_ExperienceRelatedCommission
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Percentage of comparable consolidated amount. + References No definition available. + Details
Name: | amsf_PercentageOfComparableConsolidatedAmount
Namespace Prefix: | amsf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Premium Tax Benefit Accrued + References No definition available. + Details
Name: | amsf_PremiumTaxBenefitAccrued
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | amsf_ScheduleOfAccountingPoliciesLineItems
Namespace Prefix: | amsf_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Short term investments maturity period. + References No definition available. + Details
Name: | amsf_ShortTermInvestmentsMaturityPeriod
Namespace Prefix: | amsf_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Unfunded Commitments + References No definition available. + Details
Name: | amsf_UnfundedCommitments
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount charged to advertising expense for the period, which are expenses incurred with the objective of increasing revenue for a specified brand, product or product line. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 720 -SubTopic 35 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6420018&loc=d3e36677-107848 + Details
Name: | us-gaap_AdvertisingExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_PartnerTypeOfPartnersCapitalAccountAxis=us-gaap_LimitedPartnerMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_LandAndBuildingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |