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Comprehensive Income and Accumulated Other Comprehensive Income (Loss) - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Comprehensive income tax rate	35.00%amsf_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesTaxRate

X

- Definition

Other comprehensive income loss available for sale securities tax rate.

+ References

No definition available.

+ Details

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

X

- Details

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