Comprehensive Income and Accumulated Other Comprehensive Income - Additional Information (Detail)	12 Months Ended
Dec. 31, 2016
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract]
Comprehensive income tax rate	35.00%

Other comprehensive income loss available for sale securities tax rate.

No definition available.

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