Basis of Presentation (Details)	9 Months Ended	12 Months Ended
Sep. 30, 2012	Dec. 31, 2011
Basis of Presentation (Textual) [Abstract]
Percentage of comparable consolidated amounts	99.00%	99.00%

Percentage of comparable asset and revenue consolidated amounts.

No definition available.

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