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Premiums Receivable - Summarizes the Activity in the Allowance for Doubtful Accounts (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Receivables [Line Items] |  |  |
Provision for bad debts | $ 673 us-gaap_PremiumsReceivableAllowanceForDoubtfulAccountsProvisionChargedToExpense1 | $ 52 us-gaap_PremiumsReceivableAllowanceForDoubtfulAccountsProvisionChargedToExpense1 | $ 2,230 us-gaap_PremiumsReceivableAllowanceForDoubtfulAccountsProvisionChargedToExpense1
One-Time Adjustment [Member] |  |  |
Receivables [Line Items] |  |  |
Provision for bad debts |  | $ 1,700 us-gaap_PremiumsReceivableAllowanceForDoubtfulAccountsProvisionChargedToExpense1 / us-gaap_StatementEquityComponentsAxis = amsf_OneTimeMember |

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Balance Type: | na
Period Type: | duration

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- Definition Amount of expense for known and estimated loss on collection of premiums receivable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 09 -Article 12 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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Data Type: | na
Balance Type: |
Period Type: |