Revised Expiration Date Of Share Repurchase Program

No definition available.

Name:	amsf_RevisedExpirationDateOfShareRepurchaseProgram
Namespace Prefix:	amsf_
Data Type:	xbrli:dateItemType
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Period Type:	duration

- Definition

Share Repurchase Program Date

+ References

No definition available.

+ Details

Name:	amsf_ShareRepurchaseProgramDate
Namespace Prefix:	amsf_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the declared dividend will be paid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586

+ Details

Name:	us-gaap_DividendPayableDateToBePaidDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The per share amount of a dividend declared, but not paid, as of the financial reporting date.

+ References

+ Details

Name:	us-gaap_DividendsPayableAmountPerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Date the holder must own the stock to be entitled to the dividend, in CCYY-MM-DD format.

+ References

No definition available.

+ Details

Name:	us-gaap_DividendsPayableDateOfRecordDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of a stock repurchase plan authorized by an entity's Board of Directors.

+ References

No definition available.

+ Details

Name:	us-gaap_StockRepurchaseProgramAuthorizedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_SubsequentEventLineItems
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Subsequent Events - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended	0 Months Ended
	Oct. 31, 2011	Oct. 28, 2013 Subsequent Event [Member]	Oct. 28, 2013 Subsequent Event [Member] Minimum [Member]	Oct. 28, 2013 Subsequent Event [Member] Maximum [Member]	Oct. 28, 2013 Subsequent Event [Member] Dividend Declared [Member]
Subsequent Event [Line Items]
Quarterly cash dividend per share					$ 0.08
Date of payment to shareholders entitled to dividends					Dec. 27, 2013
Date of record of shareholders entitled to dividends					Dec. 13, 2013
Share repurchase program, new limit amount	$ 25.0		$ 24.4	$ 25.0
Share repurchase program, current authorization date		Dec. 31, 2013
Share repurchase program, extended authorization date		Dec. 31, 2014