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Capital Management - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2014 | Mar. 31, 2014 | Dec. 31, 2014 | Dec. 31, 2013 | Oct. 31, 2014 | Oct. 31, 2013 | Oct. 31, 2012
Equity [Abstract] |  |  |  |  |  |  |
Share repurchase new limit amount |  |  |  |  | $ 25.0 us-gaap_StockRepurchaseProgramAuthorizedAmount1 | $ 25.0 us-gaap_StockRepurchaseProgramAuthorizedAmount1 | $ 25.0 us-gaap_StockRepurchaseProgramAuthorizedAmount1
Share repurchase program expiration date |  |  | Dec. 31, 2015 |  |  |  |
Shares repurchased | 1,258,250 us-gaap_TreasuryStockShares |  | 1,258,250 us-gaap_TreasuryStockShares | 1,258,250 us-gaap_TreasuryStockShares |  |  |
Shares repurchased, value | $ 22.4 amsf_SharePurchasedUnderShareRepurchaseProgramValue |  | $ 22.4 amsf_SharePurchasedUnderShareRepurchaseProgramValue |  |  |  |
Shares repurchased, average price |  |  | $ 17.78 amsf_ShareRepurchasesPricePaidPerShare |  |  |  |
Quarterly dividend | $ 0.12 us-gaap_DividendsPayableAmountPerShare |  | $ 0.12 us-gaap_DividendsPayableAmountPerShare | $ 0.08 us-gaap_DividendsPayableAmountPerShare |  |  |
Extraordinary cash dividends declared per common share | $ 1.000 amsf_SpecialCashDividendDeclaredPerShare | $ 0.500 amsf_SpecialCashDividendDeclaredPerShare | $ 1.50 amsf_SpecialCashDividendDeclaredPerShare | $ 0.00 amsf_SpecialCashDividendDeclaredPerShare |  |  |

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- Definition Share Purchased Under Share Repurchase Program Value. + References No definition available. + Details
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- Definition Share Repurchases Price Paid Per Share. + References No definition available. + Details
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- Definition Special cash dividend declared per share. + References No definition available. + Details
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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
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- Details
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- Definition Amount of stock repurchase plan authorized. + References No definition available. + Details
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Balance Type: | credit
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- Definition Expiration date for the purchase of an entity's own shares under a stock repurchase plan, in the CCYY-MM-DD format. + References No definition available. + Details
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- Definition Number of common and preferred shares that were previously issued and that were repurchased by the issuing entity and held in treasury on the financial statement date. This stock has no voting rights and receives no dividends. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.28,29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5 + Details
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