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v2.4.1.9

Concentration of Operations - Net Premiums Earned for the Top Ten States (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
Louisiana [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Premiums Earned Net By Segment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Premiums earned |  |  |  |  |  |  |  |  | 47,643 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_LA | 42,110 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_LA | 31,134 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_LA
Premium earned percentage |  |  |  |  |  |  |  |  | 12.70%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_LA | 12.80%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_LA | 10.70%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_LA
Georgia [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Premiums Earned Net By Segment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Premiums earned |  |  |  |  |  |  |  |  | 36,761 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_GA | 30,096 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_GA | 25,033 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_GA
Premium earned percentage |  |  |  |  |  |  |  |  | 9.80%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_GA | 9.10%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_GA | 8.60%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_GA
Pennsylvania [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Premiums Earned Net By Segment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Premiums earned |  |  |  |  |  |  |  |  | 35,488 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_PA | 28,880 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_PA | 24,480 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_PA
Premium earned percentage |  |  |  |  |  |  |  |  | 9.40%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_PA | 8.80%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_PA | 8.40%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_PA
North Carolina [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Premiums Earned Net By Segment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Premiums earned |  |  |  |  |  |  |  |  | 20,872 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_NC | 21,019 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_NC | 20,036 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_NC
Premium earned percentage |  |  |  |  |  |  |  |  | 5.60%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_NC | 6.40%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_NC | 6.90%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_NC
Florida [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Premiums Earned Net By Segment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Premiums earned |  |  |  |  |  |  |  |  | 20,691 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_FL | 12,950 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_FL | 9,180 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_FL
Premium earned percentage |  |  |  |  |  |  |  |  | 5.50%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_FL | 3.90%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_FL | 3.20%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_FL
Illinois [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Premiums Earned Net By Segment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Premiums earned |  |  |  |  |  |  |  |  | 20,606 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_IL | 18,445 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_IL | 18,119 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_IL
Premium earned percentage |  |  |  |  |  |  |  |  | 5.50%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_IL | 5.60%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_IL | 6.20%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_IL
Virginia [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Premiums Earned Net By Segment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Premiums earned |  |  |  |  |  |  |  |  | 16,764 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_VA | 15,751 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_VA | 16,144 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_VA
Premium earned percentage |  |  |  |  |  |  |  |  | 4.50%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_VA | 4.80%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_VA | 5.60%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_VA
Oklahoma [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Premiums Earned Net By Segment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Premiums earned |  |  |  |  |  |  |  |  | 16,599 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_OK | 17,473 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_OK | 16,157 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_OK
Premium earned percentage |  |  |  |  |  |  |  |  | 4.40%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_OK | 5.30%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_OK | 5.60%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_OK
Texas [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Premiums Earned Net By Segment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Premiums earned |  |  |  |  |  |  |  |  | 14,846 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_TX | 14,949 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_TX | 11,687 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_TX
Premium earned percentage |  |  |  |  |  |  |  |  | 4.00%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_TX | 4.50%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_TX | 4.00%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_TX
Minnesota [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Premiums Earned Net By Segment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Premiums earned |  |  |  |  |  |  |  |  | 13,159 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_MN | 10,470 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_MN | 10,094 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = stpr_MN
Premium earned percentage |  |  |  |  |  |  |  |  | 3.50%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_MN | 3.20%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_MN | 3.50%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = stpr_MN
All Others [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule Of Premiums Earned Net By Segment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Premiums earned |  |  |  |  |  |  |  |  | $ 132,318 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = amsf_AllOtherStatesMember | $ 117,840 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = amsf_AllOtherStatesMember | $ 108,625 us-gaap_PremiumsEarnedNet / us-gaap_StatementGeographicalAxis = amsf_AllOtherStatesMember
Premium earned percentage |  |  |  |  |  |  |  |  | 35.10%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = amsf_AllOtherStatesMember | 35.60%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = amsf_AllOtherStatesMember | 37.30%amsf_PremiumsEarnedPercentage / us-gaap_StatementGeographicalAxis = amsf_AllOtherStatesMember

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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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