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Employee Benefit Plan - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan name | 401(k) benefit plan |  |
Defined Contribution Plan, Plan Name [Extensible List] | amsf:KBenefitPlanMember |  |
Matches employee compensation for participating employees | 50.00% |  |
Fully vested in employer contributions | 5 years |  |
Defined contribution this plan | $ 0.7 | $ 0.7 | $ 0.7
Maximum [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Matches employee compensation for participating employees | 6.00% |  |

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- Definition Name of defined contribution plan designed to provide retirement benefits. + References No definition available. + Details
Name: | amsf_DefinedContributionPlanPlanName1
Namespace Prefix: | amsf_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Defined contribution plan vesting period. + References No definition available. + Details
Name: | amsf_DefinedContributionPlanVestingPeriod
Namespace Prefix: | amsf_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Indicates name of defined contribution plan designed to provide retirement benefits. Includes, but is not limited to, legal name of plan. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
Name: | us-gaap_DefinedContributionPlanPlanNameExtensibleList
Namespace Prefix: | us-gaap_
Data Type: | srt-types:extensibleListItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |