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Investments - Gross Unrealized Gains and Losses and the Amortized Cost and Fair Value of Investments Classified as Held-to-maturity (Detail) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2014 | Dec. 31, 2013
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Investment [Line Items] |  |
Amortized Cost | $ 548,612 | $ 536,583
Gross Unrealized Gains | 27,079 | 25,934
Gross Unrealized Losses | (516) | (1,338)
Fair Value | 575,175 | 561,179
States and political subdivisions [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 368,697 | 381,674
Gross Unrealized Gains | 19,970 | 18,634
Gross Unrealized Losses | (303) | (1,153)
Fair Value | 388,364 | 399,155
Corporate bonds [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 94,099 | 67,423
Gross Unrealized Gains | 945 | 861
Gross Unrealized Losses | (74) | (41)
Fair Value | 94,970 | 68,243
Commercial mortgage-backed securities [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 49,884 | 50,813
Gross Unrealized Gains | 3,079 | 3,431
Fair Value | 52,963 | 54,244
U.S. agency-based mortgage-backed securities [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 20,418 | 21,775
Gross Unrealized Gains | 1,794 | 1,790
Fair Value | 22,212 | 23,565
U.S. Treasury securities and obligations of U.S. government agencies [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 12,265 | 11,514
Gross Unrealized Gains | 1,077 | 1,002
Gross Unrealized Losses | (6) |
Fair Value | 13,336 | 12,516
Asset-backed securities [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 3,249 | 3,384
Gross Unrealized Gains | 214 | 216
Gross Unrealized Losses | (133) | (144)
Fair Value | $ 3,330 | $ 3,456

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- Definition This item represents the excess of amortized cost basis over fair value of securities in a gain position and which are categorized as held-to-maturity. + References No definition available. + Details
Name: | amsf_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the excess of amortized cost basis over fair value of securities in a loss position and which are categorized as held-to-maturity. + References No definition available. + Details
Name: | amsf_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Namespace Prefix: | amsf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | invest_InvestmentLineItems
Namespace Prefix: | invest_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.6) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | debit
Period Type: | instant

X
- Definition Fair value of investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant