Share Purchased Under Share Repurchase Program Value.

No definition available.

Name:	amsf_SharePurchasedUnderShareRepurchaseProgramValue
Namespace Prefix:	amsf_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Share Repurchases Price Paid Per Share.

+ References

No definition available.

+ Details

Name:	amsf_ShareRepurchasesPricePaidPerShare
Namespace Prefix:	amsf_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Special cash dividend declared per share.

+ References

No definition available.

+ Details

Name:	amsf_SpecialCashDividendDeclaredPerShare
Namespace Prefix:	amsf_
Data Type:	num:perShareItemType
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

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

- Details

Name:	us-gaap_EquityAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of stock repurchase plan authorized.

+ References

No definition available.

+ Details

Name:	us-gaap_StockRepurchaseProgramAuthorizedAmount1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Expiration date for the purchase of an entity's own shares under a stock repurchase plan, in the CCYY-MM-DD format.

+ References

No definition available.

+ Details

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

- Definition

Number of common and preferred shares that were previously issued and that were repurchased by the issuing entity and held in treasury on the financial statement date. This stock has no voting rights and receives no dividends.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.28,29)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 29, 30

-Article 5

+ Details

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

Capital Management - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Oct. 31, 2014	Oct. 31, 2013	Oct. 31, 2012
Equity [Abstract]
Share repurchase new limit amount					$ 25.0	$ 25.0	$ 25.0
Share repurchase program expiration date			Dec. 31, 2015
Shares repurchased	1,258,250us-gaap_TreasuryStockShares		1,258,250us-gaap_TreasuryStockShares	1,258,250us-gaap_TreasuryStockShares
Shares repurchased, value	$ 22.4		$ 22.4
Shares repurchased, average price			$ 17.78amsf_ShareRepurchasesPricePaidPerShare
Quarterly dividend	$ 0.12us-gaap_DividendsPayableAmountPerShare		$ 0.12us-gaap_DividendsPayableAmountPerShare	$ 0.08us-gaap_DividendsPayableAmountPerShare
Extraordinary cash dividends declared per common share	$ 1.000	$ 0.500	$ 1.50	$ 0.00