Stock options exercise price.

No definition available.

Name:	amsf_ExercisePriceAsPercentageOfFairMarketValueOfUnderlyingSharesOnDateOfGrant
Namespace Prefix:	amsf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Stock options exercise price.

+ References

No definition available.

+ Details

Name:	amsf_ExercisePriceAsPercentageOfFairMarketValueOfUnderlyingSharesOnDateOfGrantOne
Namespace Prefix:	amsf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of companies used for the 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

- 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

- 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

- Definition

Expiration of stock option.

+ References

No definition available.

+ Details

Name:	amsf_ShareBasedCompensationArrangementByShareBasedPaymentAwardExpirationPeriod
Namespace Prefix:	amsf_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Company's voting stock.

+ References

No definition available.

+ Details

Name:	amsf_ShareBasedCompensationArrangementGrantRestrictionVotingPercentage
Namespace Prefix:	amsf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	amsf_StockOptionsAndRestrictedStockTextualAbstract
Namespace Prefix:	amsf_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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 2

-Subparagraph (h)(1)(i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph g(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: 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 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section F

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph 64

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: 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=6793087&loc=d3e301413-122809

+ Details

Name:	us-gaap_AllocatedShareBasedCompensationExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of excess tax benefit (tax deficiency) that arises when compensation cost from non-qualified share-based compensation recognized on the entity's tax return exceeds (is less than) compensation cost from equity-based compensation recognized in financial statements. Excess tax benefit (tax deficiency) increases (decreases) net cash provided by financing activities.

+ 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=6576910&loc=d3e11374-113907

+ Details

Name:	us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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=7578670&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssumptionsExpectedDividendRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- 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

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- 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 Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: 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

- 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

- Definition

Gross number of share options (or share units) granted during the period.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Stock Options and Restricted Stock (Details Textual) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Stock Options and Restricted Stock (Textual) [Abstract]
Options granted under the 2005 Incentive Plan
Total tax expense	421,000	1,043,000	$ 238,000
2010 Restricted Stock Plan [Member]
Stock Options and Restricted Stock (Textual) [Abstract]
Shares authorized under incentive plan	100,000
Restricted stock outstanding	7,833	9,513	11,830	5,844
Total tax expense			5,000
Restricted stock granted	7,833	9,513	12,246
2012 Equity and Incentive Compensation Plan [Member]
Stock Options and Restricted Stock (Textual) [Abstract]
Shares authorized under incentive plan	500,000
Common stock available for future awards	497,669
Incentive Plan [Member]
Stock Options and Restricted Stock (Textual) [Abstract]
Stock options exercise price	100.00%
Stock options exercise price	110.00%
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
Dividend yield assumed	0.00%
Common stock available for future awards	248,009
Restricted stock outstanding	8,000
Compensation expense	244,000	261,000	2,162,000
Restricted stock granted	8,000
Incentive Plan [Member] \| Minimum [Member]
Stock Options and Restricted Stock (Textual) [Abstract]
Company's voting stock	10.00%
2005 Incentive Plan [Member]
Stock Options and Restricted Stock (Textual) [Abstract]
Options granted under the 2005 Incentive Plan		0	0
Restricted Stock Units [Member]
Stock Options and Restricted Stock (Textual) [Abstract]
Common stock available for future awards	70,824
Restricted stock award for a number of shares	30,000
Restricted stock outstanding	7,833
Compensation expense	210,000	210,000	163,000
Total tax expense	16,000	18,000
Restricted Stock Units [Member] \| 2012 Equity and Incentive Compensation Plan [Member]
Stock Options and Restricted Stock (Textual) [Abstract]
Restricted stock outstanding	2,331
Restricted stock granted	2,331
Share Based Grants [Member] \| 2012 Equity and Incentive Compensation Plan [Member]
Stock Options and Restricted Stock (Textual) [Abstract]
Compensation expense	12,000
Long-Term Incentive Awards [Member] \| 2012 Equity and Incentive Compensation Plan [Member]
Stock Options and Restricted Stock (Textual) [Abstract]
Compensation expense	175,000