XML 104 R80.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Stock Options and Restricted Stock - Additional Information (Detail) (USD $) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Total tax expense | $ 2,841,000 us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities | $ 2,024,000 us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities | $ 421,000 us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities |
2005 Incentive Plan [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Stock options exercise price | 100.00%amsf_ExercisePriceAsPercentageOfFairMarketValueOfUnderlyingSharesOnDateOfGrant / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember |  |  |
Company's voting stock | 10.00%amsf_ShareBasedCompensationArrangementGrantRestrictionVotingPercentage / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember |  |  |
Stock options exercise price | 110.00%amsf_ExercisePriceAsPercentageOfFairMarketValueOfUnderlyingSharesOnDateOfGrantOne / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember |  |  |
Duration for Stock option grants exercisable | 10 years |  |  |
Stock options exercise period | 5 years |  |  |
Expiration of stock option | 90 days |  |  |
Unexercised portion of stock option remains exercisable | 1 year |  |  |
Number of companies used for the estimation of expected volatility | 6 amsf_NumberOfCompaniesUsedForEstimationOfExpectedVolatility / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember |  |  |
Dividend yield assumed | 0.00%us-gaap_FairValueAssumptionsExpectedDividendRate / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember |  |  |
Cash dividends paid |  | 0 us-gaap_DividendsShareBasedCompensationCash / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember |  |
Compensation expense | 300,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember | 300,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember | 200,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember |
Total tax expense | 38,000 us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember | 4,000 us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember |  |
Restricted stock outstanding | 2,400 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember | 6,400 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember | 8,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember |
Restricted stock granted |  |  | 8,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = amsf_TwoThousandFiveIncentivePlanMember |
2010 Restricted Stock Plan [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation expense | 200,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember | 200,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember | 200,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember |
Total tax expense | 7,000 us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember | 18,000 us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember | 16,000 us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember |
Shares authorized under incentive plan | 100,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember |  |  |
Common stock available for future awards | 33,186 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember |  |  |
Restricted stock award for a number of shares | 30,000 amsf_RestrictedStockAwardForNumberOfStocks / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember |  |  |
Restricted stock outstanding | 5,229 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember | 5,376 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember | 7,833 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember | 9,513 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember
Restricted stock granted | 5,229 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember | 5,376 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember | 7,833 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = amsf_TwoThousandTenRestrictedStockPlanMember |
2012 Equity and Incentive Compensation Plan [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Shares authorized under incentive plan | 500,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember |  |  |
Common stock available for future awards | 413,079 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_PlanNameAxis = amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember |  |  |
Restricted stock granted | 4,312 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_PlanNameAxis = amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember |  |  |
2012 Equity and Incentive Compensation Plan [Member] | Share Based Grants [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation expense | 600,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = amsf_ShareBasedGrantsMember / us-gaap_PlanNameAxis = amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember | 300,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = amsf_ShareBasedGrantsMember / us-gaap_PlanNameAxis = amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember | 12,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = amsf_ShareBasedGrantsMember / us-gaap_PlanNameAxis = amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember |
2012 Equity and Incentive Compensation Plan [Member] | Long-Term Incentive Awards [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation expense | $ 400,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = amsf_LongTermIncentiveAwardsMember / us-gaap_PlanNameAxis = amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember | $ 600,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = amsf_LongTermIncentiveAwardsMember / us-gaap_PlanNameAxis = amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember | $ 200,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = amsf_LongTermIncentiveAwardsMember / us-gaap_PlanNameAxis = amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember |
2012 Equity and Incentive Compensation Plan [Member] | Restricted Stock Units (RSUs) [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Restricted stock granted |  | 80,278 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember / us-gaap_PlanNameAxis = amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember |  |

X
- Definition Exercise Price As Percentage Of Fair Market Value Of Underlying Shares On Date Of Grant + References No definition available. + Details
Name: | amsf_ExercisePriceAsPercentageOfFairMarketValueOfUnderlyingSharesOnDateOfGrant
Namespace Prefix: | amsf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Exercise Price As Percentage Of Fair Market Value Of Underlying Shares On Date Of Grant One + References No definition available. + Details
Name: | amsf_ExercisePriceAsPercentageOfFairMarketValueOfUnderlyingSharesOnDateOfGrantOne
Namespace Prefix: | amsf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number Of Companies Used For Estimation Of Expected Volatility + References No definition available. + Details
Name: | amsf_NumberOfCompaniesUsedForEstimationOfExpectedVolatility
Namespace Prefix: | amsf_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Restricted Stock Award For Number Of Stocks + References No definition available. + Details
Name: | amsf_RestrictedStockAwardForNumberOfStocks
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- Definition Share Based Compensation Arrangement By Share Based Payment Award Award Requisite Service Period Remain Exercisable + References No definition available. + Details
Name: | amsf_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriodRemainExercisable
Namespace Prefix: | amsf_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Share Based Compensation Arrangement Grant Restriction Voting Percentage + References No definition available. + Details
Name: | amsf_ShareBasedCompensationArrangementGrantRestrictionVotingPercentage
Namespace Prefix: | amsf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
Name: | us-gaap_AllocatedShareBasedCompensationExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of paid and unpaid cash dividends declared and applicable to holders of share-based compensation, for example, but not limited to, non-vested shares, stock options, or restricted stock units. + References No definition available. + Details
Name: | us-gaap_DividendsShareBasedCompensationCash
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of cash inflow from realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 20 -Section 55 -Paragraph 24 -URI http://asc.fasb.org/extlink&oid=32706628&loc=d3e11374-113907 + Details
Name: | us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Expected dividends to be paid to holders of the underlying shares or financial instruments (expressed as a percentage of the share or instrument's price). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueAssumptionsExpectedDividendRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_PlanNameAxis=amsf_TwoThousandFiveIncentivePlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PlanNameAxis=amsf_TwoThousandTenRestrictedStockPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PlanNameAxis=amsf_TwoThousandTwelveEquityAndIncentiveCompensationPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |