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Subsequent Events - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified | 0 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Feb. 11, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2015 | Feb. 24, 2015
Subsequent Event [Line Items] |  |  |  |  |
Reserve for the claim |  | $ 2.7 us-gaap_LitigationReserve |  |  |
Additional reserve for the claim |  | 0.3 us-gaap_LitigationReserveCurrent |  |  |
Cash dividend per share |  | $ 0.12 us-gaap_DividendsPayableAmountPerShare | $ 0.08 us-gaap_DividendsPayableAmountPerShare |  |
Scenario, Forecast [Member] |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Expected cash dividend per share |  |  |  | $ 0.60 amsf_ExpectedCashDividendPerShare / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |
Subsequent Event [Member] |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Total compensation claim | 25.3 us-gaap_LitigationSettlementAmount / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |  |  |  |
Subsequent Event [Member] | Plaintiff Seeking Actual Damages [Member] |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Amount of damages | 0.3 us-gaap_LossContingencyDamagesAwardedValue / us-gaap_LossContingenciesByNatureOfContingencyAxis = amsf_PlaintiffSeekingActualDamagesMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |  |  |  |
Subsequent Event [Member] | Plaintiff Seeking Punitive Damages [Member] |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Amount of damages | $ 25.0 us-gaap_LossContingencyDamagesAwardedValue / us-gaap_LossContingenciesByNatureOfContingencyAxis = amsf_PlaintiffSeekingPunitiveDamagesMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |  |  |  |
Dividend Declared [Member] |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Cash dividend per share |  | $ 0.12 us-gaap_DividendsPayableAmountPerShare / us-gaap_DividendsAxis = us-gaap_DividendDeclaredMember |  |  |
Date of payment to shareholders entitled to dividends |  | Mar. 27, 2015 |  |  |
Date of record of shareholders entitled to dividends |  | Mar. 13, 2015 |  |  |
Dividend Declared [Member] | Subsequent Event [Member] |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Cash dividend per share |  |  |  |  | $ 0.15 us-gaap_DividendsPayableAmountPerShare / us-gaap_DividendsAxis = us-gaap_DividendDeclaredMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

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- Definition Expected Cash Dividend Per Share + References No definition available. + Details
Name: | amsf_ExpectedCashDividendPerShare
Namespace Prefix: | amsf_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

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- Definition Date the declared dividend will be paid, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586 + Details
Name: | us-gaap_DividendPayableDateToBePaidDayMonthAndYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition The per share amount of a dividend declared, but not paid, as of the financial reporting date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586 + Details
Name: | us-gaap_DividendsPayableAmountPerShare
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

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- Definition Date the holder must own the stock to be entitled to the dividend, in CCYY-MM-DD format. + References No definition available. + Details
Name: | us-gaap_DividendsPayableDateOfRecordDayMonthAndYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition Aggregate carrying amount of the estimated litigation liability for known or estimated probable loss from litigation, which may include attorneys' fees and other litigation costs. + 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 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.25) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LitigationReserve
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Carrying amount of reserve for known or estimated probable loss from litigation, which may include attorneys' fees and other litigation costs, which is expected to be paid within one year of the date of the statement of financial position. + 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 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.25) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 25 -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 55 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=6396582&loc=d3e15243-108350 + Details
Name: | us-gaap_LitigationReserveCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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

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

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- Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_DividendsAxis=us-gaap_DividendDeclaredMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |