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Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Insurance [Abstract]
Paid losses recoverable outstanding due date	90 days
Received reinsurance recoveries	$ 0.3	$ 16.0	$ 1.4
Unsecured reinsurance recoverable by reinsurer reporting threshold	1.50%

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- Definition

Paid Loss Recoverable Past Due Period

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No definition available.

+ Details

Name:	amsf_PaidLossRecoverablePastDuePeriod
Namespace Prefix:	amsf_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Threshold For Reporting Unsecured Reinsurance Recoverables.

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No definition available.

+ Details

Name:	amsf_PercentageLimitForReportingUnsecuredReinsuranceRecoverables
Namespace Prefix:	amsf_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Proceeds From Reinsurance Recoveries

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No definition available.

+ Details

Name:	amsf_ProceedsFromReinsuranceRecoveries
Namespace Prefix:	amsf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- References

No definition available.

+ Details

Name:	us-gaap_InsuranceAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration