XML 42 R32.htm IDEA: XBRL DOCUMENT
v3.19.1

Investments - Gross Unrealized Gains and Losses and Cost of Equity (Detail) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Investment [Line Items] |  |
Equity securities, Cost | $ 20,769 | $ 19,962
Equity securities, Gross Unrealized Gains | 846 | 30
Equity securities, Gross Unrealized Losses |  | (1,341)
Equity securities, Fair Value | 21,615 | 18,651
Domestic Common Stock [Member] |  |
Investment [Line Items] |  |
Equity securities, Cost | 20,769 | 19,962
Equity securities, Gross Unrealized Gains | 846 | 30
Equity securities, Gross Unrealized Losses |  | (1,341)
Equity securities, Fair Value | $ 21,615 | $ 18,651

X
- Definition Equity securities fair value net income gross unrealized gain. + References No definition available. + Details
Name: | amsf_EquitySecuritiesFvNiGrossUnrealizedGain
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Equity Securities fair value net income gross unrealized loss. + References No definition available. + Details
Name: | amsf_EquitySecuritiesFvNiGrossUnrealizedLoss
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | invest_InvestmentLineItems
Namespace Prefix: | invest_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740 + Details
Name: | us-gaap_EquitySecuritiesFvNi
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value. + References No definition available. + Details
Name: | us-gaap_EquitySecuritiesFvNiCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=amsf_DomesticCommonStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |