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Statutory Accounting and Regulatory Requirements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Insurance [Abstract] |  |  |
Dividends to the Company | 10.00%amsf_StatutoryAccountingPracticesFutureDividendPaymentsPercentage |  |
Dividends paid | $ 25.0 us-gaap_PaymentsOfDividends | $ 15.0 us-gaap_PaymentsOfDividends | $ 21.0 us-gaap_PaymentsOfDividends
Dividends permissable to pay without seeking regulatory approval | $ 37.8 us-gaap_StatutoryAccountingPracticesStatutoryAmountAvailableForDividendPayments |  |

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- Definition Statutory Accounting Practices Future Dividend Payments Percentage + References No definition available. + Details
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- Details
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- Definition Cash outflow in the form of capital distributions and dividends to common shareholders, preferred shareholders and noncontrolling interests. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585 + Details
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- Definition Amount of dividends available for payment after computation of statutory restrictions on capital surplus. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(e)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph e -Article 4 + Details
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Period Type: | instant