XML 84 R94.htm IDEA: XBRL DOCUMENT

Concentration of Operations - Net Premiums Earned for the Different Lines of Business (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Premiums Earned Net By Segment [Line Items]
Premiums earned	$ 97,070us-gaap_PremiumsEarnedNet	$ 95,928us-gaap_PremiumsEarnedNet	$ 93,516us-gaap_PremiumsEarnedNet	$ 89,233us-gaap_PremiumsEarnedNet	$ 86,695us-gaap_PremiumsEarnedNet	$ 81,596us-gaap_PremiumsEarnedNet	$ 81,983us-gaap_PremiumsEarnedNet	$ 79,709us-gaap_PremiumsEarnedNet	$ 375,747us-gaap_PremiumsEarnedNet	$ 329,983us-gaap_PremiumsEarnedNet	$ 290,689us-gaap_PremiumsEarnedNet
Premium earned percentage									100.00%amsf_PremiumsEarnedPercentage	100.00%amsf_PremiumsEarnedPercentage	100.00%amsf_PremiumsEarnedPercentage
Workers Compensation [Member]
Schedule Of Premiums Earned Net By Segment [Line Items]
Premiums earned									375,746	329,986	290,538
Premium earned percentage									100.00%	100.00%	99.90%
General Liability [Member]
Schedule Of Premiums Earned Net By Segment [Line Items]
Premiums earned									$ 1	$ (3)	$ 151
Premium earned percentage									0.00%	0.00%	0.10%

- Definition

Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

+ References+ Details