XML 17 R24.htm IDEA: XBRL DOCUMENT
v2.4.0.8

Basis of Presentation - Additional Information (Detail) | 6 Months Ended
--- | ---
Jun. 30, 2013 | Jun. 30, 2012
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