Comprehensive Income and Accumulated Other Comprehensive Income (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Reclassification adjustments related to unrealized gains on investment securities:
Having no credit losses recognized in the consolidated statement of income (1)	$ (22)	$ (343)
Income tax expense (2)	(8)	(120)
Total reclassifications, net of taxes	$ (14)	$ (223)

Other comprehensive income loss reclassification adjustment for investment securities having no credit losses included in net income net of tax.

No definition available.

Name:	amsf_OtherComprehensiveIncomeLossReclassificationAdjustmentForInvestmentSecuritiesHavingNoCreditLossesIncludedInNetIncomeNetOfTax
Namespace Prefix:	amsf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration