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Employee Benefit Plan - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract] |  |  |
Defined contribution plan name | 401(k) benefit plan |  |
Matches employee compensation for participating employees | 3.00% |  |
Fully vested in employer contributions | 5 years |  |
Defined contribution this plan | $ 0.6 | $ 0.4 | $ 0.4

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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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- References No definition available. + Details
Name: | us-gaap_CompensationAndRetirementDisclosureAbstract
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

X
- 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 Name of defined contribution plan designed to provide retirement benefits. Includes, but is not limited to, legal name of plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanPlanName
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration