Appeal bond.

No definition available.

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

- 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 of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

+ References

No definition available.

+ Details

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

- Definition

Amount of judgment or settlement awarded to (against) the entity in respect of litigation.

+ References

No definition available.

+ Details

Name:	us-gaap_LitigationSettlementAmount
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=25496072&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=25496072&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=25496072&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
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Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Feb. 28, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Loss Contingencies [Line Items]
Rent expense		$ 0.2	$ 0.2	$ 0.2
Retention amount		5.0
American Interstate Insurance Company [Member] \| Iowa [Member]
Loss Contingencies [Line Items]
Appeal bond		27.8
American Interstate Insurance Company [Member] \| 2009 Workers' Compensation Claim [Member] \| Iowa [Member]
Loss Contingencies [Line Items]
Workers compensation claim, damages awarded	$ 25.3
Workers compensation claims contingency reserves		2.5
Total amount expensed on workers compensation claim		$ 5.4
American Interstate Insurance Company [Member] \| Plaintiff Seeking Actual Damages [Member] \| Iowa [Member]
Loss Contingencies [Line Items]
Amount of damages	0.3
American Interstate Insurance Company [Member] \| Plaintiff Seeking Punitive Damages [Member] \| Iowa [Member]
Loss Contingencies [Line Items]
Amount of damages	$ 25.0