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Reinsurance - Schedule of Unsecured Reinsurance Recoverables from Reinsurers (Detail) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2013 | Dec. 31, 2012
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Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | $ 75,326 | $ 101,352
Total unsecured reinsurance recoverables | 35,993 |
Letters of credit and funds held [Member] |  |
Ceded Credit Risk [Line Items] |  |
Letters of credit and funds held | (39,333) |
Ceded Credit Risk, Concentrated Credit Risk [Member] | Hannover Reinsurance (Ireland) Limited (A+) [Member] |  |
Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | 23,956 |
Ceded Credit Risk, Concentrated Credit Risk [Member] | Odyssey America Reinsurance Corporation (A) [Member] |  |
Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | 12,045 |
Ceded Credit Risk, Concentrated Credit Risk [Member] | Minnesota Workers' Compensation Reinsurance Association (NR) [Member] |  |
Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | 6,784 |
Ceded Credit Risk, Concentrated Credit Risk [Member] | Tokio Millenium Reinsurance Limited (A++) [Member] |  |
Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | 6,246 |
Ceded Credit Risk, Concentrated Credit Risk [Member] | Clearwater Insurance Company (bbb) [Member] |  |
Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | 5,693 |
Ceded Credit Risk, Concentrated Credit Risk [Member] | Other reinsurers [Member] |  |
Ceded Credit Risk [Line Items] |  |
Total reinsurance recoverables | $ 20,602 |

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- Definition Letters of credit from the reinsurer and Amount of the reinsurer's funds held on its behalf by the ceding insurer, including funds held in trust, under reinsurance treaties or contracts with the assuming insurer as security for the payment of the obligations thereunder. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Unsecured Reinsurance Recoverables Net + References No definition available. + Details
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Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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- Definition Amount after valuation allowance recoverable including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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