Appeal bond.

No definition available.

Name:	amsf_AppealBond
Namespace Prefix:	amsf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Appeal bond returned amount.

+ References

No definition available.

+ Details

Name:	amsf_AppealBondReturnedAmount
Namespace Prefix:	amsf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Workers compensation claims contingency reserves.

+ References

No definition available.

+ Details

Name:	amsf_WorkersCompensationClaimsContingencyReserves
Namespace Prefix:	amsf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Workers compensation claims contingency reserves plus inception to date loss and loss adjustment expense paid.

+ References

No definition available.

+ Details

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

- Definition

Amount awarded to other party in judgment or settlement of litigation.

+ References

No definition available.

+ Details

Name:	us-gaap_LitigationSettlementAmountAwardedToOtherParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

- Definition

Amount of damages awarded to the plaintiff in the legal matter.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14435-108349

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14557-108349

+ Details

Name:	us-gaap_LossContingencyDamagesAwardedValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Stated amount that is the upper retention limit per entity policy.

+ References

No definition available.

+ Details

Name:	us-gaap_ReinsuranceRetentionPolicyAmountRetained
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	dei_LegalEntityAxis=amsf_AmericanInterstateInsuranceCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_IA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=amsf_TwoThousandAndNineWorkersCompensationClaimMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=amsf_PlaintiffSeekingActualDamagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=amsf_PlaintiffSeekingPunitiveDamagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions		1 Months Ended	9 Months Ended
	Jul. 24, 2017	Feb. 28, 2015	Sep. 30, 2017
Loss Contingencies [Line Items]
Retention amount			$ 5.0
AIIC [Member]
Loss Contingencies [Line Items]
Appeal bond returned amount	$ 27.8
AIIC [Member] \| Iowa [Member]
Loss Contingencies [Line Items]
Appeal bond			27.8
AIIC [Member] \| 2009 Workers' Compensation Claim [Member] \| Iowa [Member]
Loss Contingencies [Line Items]
Workers compensation claim, damages awarded		$ 25.3
Workers compensation claims contingency reserves			2.3
Total amount expensed on workers compensation claim			$ 5.4
AIIC [Member] \| Plaintiff Seeking Actual Damages [Member] \| Iowa [Member]
Loss Contingencies [Line Items]
Amount of damages		0.3
AIIC [Member] \| Plaintiff Seeking Punitive Damages [Member] \| Iowa [Member]
Loss Contingencies [Line Items]
Amount of damages		$ 25.0