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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
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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 | $ 97,070 us-gaap_PremiumsEarnedNet | $ 95,928 us-gaap_PremiumsEarnedNet | $ 93,516 us-gaap_PremiumsEarnedNet | $ 89,233 us-gaap_PremiumsEarnedNet | $ 86,695 us-gaap_PremiumsEarnedNet | $ 81,596 us-gaap_PremiumsEarnedNet | $ 81,983 us-gaap_PremiumsEarnedNet | $ 79,709 us-gaap_PremiumsEarnedNet | $ 375,747 us-gaap_PremiumsEarnedNet | $ 329,983 us-gaap_PremiumsEarnedNet | $ 290,689 us-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 us-gaap_PremiumsEarnedNet / us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis = amsf_WorkersCompensationMember | 329,986 us-gaap_PremiumsEarnedNet / us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis = amsf_WorkersCompensationMember | 290,538 us-gaap_PremiumsEarnedNet / us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis = amsf_WorkersCompensationMember
Premium earned percentage |  |  |  |  |  |  |  |  | 100.00%amsf_PremiumsEarnedPercentage / us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis = amsf_WorkersCompensationMember | 100.00%amsf_PremiumsEarnedPercentage / us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis = amsf_WorkersCompensationMember | 99.90%amsf_PremiumsEarnedPercentage / us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis = amsf_WorkersCompensationMember
General Liability [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Premiums Earned Net By Segment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Premiums earned |  |  |  |  |  |  |  |  | $ 1 us-gaap_PremiumsEarnedNet / us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis = us-gaap_GeneralLiabilityMember | $ (3)us-gaap_PremiumsEarnedNet / us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis = us-gaap_GeneralLiabilityMember | $ 151 us-gaap_PremiumsEarnedNet / us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis = us-gaap_GeneralLiabilityMember
Premium earned percentage |  |  |  |  |  |  |  |  | 0.00%amsf_PremiumsEarnedPercentage / us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis = us-gaap_GeneralLiabilityMember | 0.00%amsf_PremiumsEarnedPercentage / us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis = us-gaap_GeneralLiabilityMember | 0.10%amsf_PremiumsEarnedPercentage / us-gaap_ScheduleOfMalpracticeInsuranceTypeAndTierIdentifierAxis = us-gaap_GeneralLiabilityMember

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- Definition Premiums Earned Percentage + References No definition available. + Details
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- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(c)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
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