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Investment - Major Categories of the Company's Net Investment Income (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total gross investment income |  |  |  |  |  |  |  |  | $ 28,891 amsf_GrossInvestmentIncome | $ 28,402 amsf_GrossInvestmentIncome | $ 28,102 amsf_GrossInvestmentIncome
Investment expenses |  |  |  |  |  |  |  |  | (1,677)us-gaap_InvestmentIncomeInvestmentExpense | (1,373)us-gaap_InvestmentIncomeInvestmentExpense | (1,084)us-gaap_InvestmentIncomeInvestmentExpense
Net investment income | 7,166 us-gaap_NetInvestmentIncome | 6,495 us-gaap_NetInvestmentIncome | 6,845 us-gaap_NetInvestmentIncome | 6,708 us-gaap_NetInvestmentIncome | 6,763 us-gaap_NetInvestmentIncome | 6,947 us-gaap_NetInvestmentIncome | 6,649 us-gaap_NetInvestmentIncome | 6,670 us-gaap_NetInvestmentIncome | 27,214 us-gaap_NetInvestmentIncome | 27,029 us-gaap_NetInvestmentIncome | 27,018 us-gaap_NetInvestmentIncome
Fixed Maturity Securities Available for Sale [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total gross investment income |  |  |  |  |  |  |  |  | 26,622 amsf_GrossInvestmentIncome / us-gaap_InvestmentTypeAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember | 26,422 amsf_GrossInvestmentIncome / us-gaap_InvestmentTypeAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember | 27,167 amsf_GrossInvestmentIncome / us-gaap_InvestmentTypeAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember
Short-term Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total gross investment income |  |  |  |  |  |  |  |  | 426 amsf_GrossInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember | 634 amsf_GrossInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember | 489 amsf_GrossInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember
Equity Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total gross investment income |  |  |  |  |  |  |  |  | 232 amsf_GrossInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember | 543 amsf_GrossInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember | 446 amsf_GrossInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember
Other Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total gross investment income |  |  |  |  |  |  |  |  | $ 1,611 amsf_GrossInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember | $ 803 amsf_GrossInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember |

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- Definition Amount, before investment expense, of income earned from investments in securities, real estate, and alternate investments. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments. + References No definition available. + Details
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- Definition Amount of expenses related to the generation of investment income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.2) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.9) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
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- Definition Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 16 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.2) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 2 -Article 7 + Details
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