XML 78 R53.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Investments - A Summary of the Company's Realized Gains and Losses on Sales, Calls or Redemptions of Investments (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
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] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  | $ 10,548 us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities | $ 10,990 us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities | $ 33,140 us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities
Gross realized investment gains |  |  |  |  |  |  |  |  | 750 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains | 1,354 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains | 2,741 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Gross realized investment (losses) |  |  |  |  |  |  |  |  | (451)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses | (471)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses |
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 299 us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments | 883 us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments | 2,741 us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments
Impairments |  |  |  |  |  |  |  |  | (222)us-gaap_ImpairmentOfInvestments |  |
Impairments |  |  |  |  |  |  |  |  |  | (2,229)us-gaap_AssetImpairmentCharges |
Other, including gains on calls and redemptions |  |  |  |  |  |  |  |  | 620 amsf_OtherGainLossIncludingGainsOnCallsAndRedemptions | 135 amsf_OtherGainLossIncludingGainsOnCallsAndRedemptions | 238 amsf_OtherGainLossIncludingGainsOnCallsAndRedemptions
Net realized investment gains (losses) | 516 us-gaap_MarketableSecuritiesRealizedGainLoss | (152)us-gaap_MarketableSecuritiesRealizedGainLoss | 232 us-gaap_MarketableSecuritiesRealizedGainLoss | 101 us-gaap_MarketableSecuritiesRealizedGainLoss | 710 us-gaap_MarketableSecuritiesRealizedGainLoss | (654)us-gaap_MarketableSecuritiesRealizedGainLoss | (1,291)us-gaap_MarketableSecuritiesRealizedGainLoss | 24 us-gaap_MarketableSecuritiesRealizedGainLoss | 697 us-gaap_MarketableSecuritiesRealizedGainLoss | (1,211)us-gaap_MarketableSecuritiesRealizedGainLoss | 2,979 us-gaap_MarketableSecuritiesRealizedGainLoss
Fixed Maturity Securities Available for Sale [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  | 768 us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember | 2,090 us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember | 22,363 us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember
Gross realized investment gains |  |  |  |  |  |  |  |  | 1 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember | 90 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember | 1,555 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 1 us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember | 90 us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember | 1,555 us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember
Impairments |  |  |  |  |  |  |  |  | (222)us-gaap_ImpairmentOfInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember |  |
Other, including gains on calls and redemptions |  |  |  |  |  |  |  |  | 244 amsf_OtherGainLossIncludingGainsOnCallsAndRedemptions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember | 38 amsf_OtherGainLossIncludingGainsOnCallsAndRedemptions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember | 15 amsf_OtherGainLossIncludingGainsOnCallsAndRedemptions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 23 us-gaap_MarketableSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember | 128 us-gaap_MarketableSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember | 1,570 us-gaap_MarketableSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_FixedMaturitySecuritiesAvailableForSaleMember
Equity Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sales |  |  |  |  |  |  |  |  | 9,780 us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 8,900 us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 10,777 us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Gross realized investment gains |  |  |  |  |  |  |  |  | 749 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 1,264 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 1,186 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Gross realized investment (losses) |  |  |  |  |  |  |  |  | (451)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | (471)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 298 us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 793 us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 1,186 us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Impairments |  |  |  |  |  |  |  |  |  | (2,229)us-gaap_AssetImpairmentCharges / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | 298 us-gaap_MarketableSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | (1,436)us-gaap_MarketableSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 1,186 us-gaap_MarketableSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Other [Member] |  |  |  |  |  |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other, including gains on calls and redemptions |  |  |  |  |  |  |  |  | 376 amsf_OtherGainLossIncludingGainsOnCallsAndRedemptions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_Other1Member | 97 amsf_OtherGainLossIncludingGainsOnCallsAndRedemptions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_Other1Member | 223 amsf_OtherGainLossIncludingGainsOnCallsAndRedemptions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_Other1Member
Net realized investment gains (losses) |  |  |  |  |  |  |  |  | $ 376 us-gaap_MarketableSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_Other1Member | $ 97 us-gaap_MarketableSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_Other1Member | $ 223 us-gaap_MarketableSecuritiesRealizedGainLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = amsf_Other1Member

X
- Definition Other Gain Loss Including Gains On Calls And Redemptions + References No definition available. + Details
Name: | amsf_OtherGainLossIncludingGainsOnCallsAndRedemptions
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | invest_InvestmentLineItems
Namespace Prefix: | invest_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=8077374&loc=d3e2420-110228 + Details
Name: | us-gaap_AssetImpairmentCharges
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26805-111562 + Details
Name: | us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
Name: | us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
Name: | us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25351-111560 + Details
Name: | us-gaap_ImpairmentOfInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition This item represents the total realized gain (loss) included in earnings for the period as a result of selling marketable securities categorized as trading, available-for-sale, or held-to-maturity. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 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 210.5-03.7(c)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27405-111563 + Details
Name: | us-gaap_MarketableSecuritiesRealizedGainLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The cash inflow associated with the aggregate amount received by the entity through sale or maturity of marketable securities (held-to-maturity or available-for-sale) during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26853-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585 + Details
Name: | us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=amsf_FixedMaturitySecuritiesAvailableForSaleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=amsf_Other1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |