XML 58 R22.htm IDEA: XBRL DOCUMENT v2.4.0.8

Basis of Presentation - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Percentage of comparable consolidated amounts	95.00%	95.00%

X

- Definition

Percentage of comparable consolidated amount.

+ References

No definition available.

+ Details

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

X

- Details

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