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Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-maturity (Detail) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2013 | Dec. 31, 2012
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Investment [Line Items] |  |
Amortized Cost | $ 550,153 | $ 583,287
Gross Unrealized Gains | 28,388 | 44,361
Gross Unrealized Losses | (1,552) | (299)
Fair Value | 576,989 | 627,349
States and political subdivisions [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 388,694 | 400,687
Gross Unrealized Gains | 20,250 | 31,387
Gross Unrealized Losses | (1,362) | (106)
Fair Value | 407,582 | 431,968
Corporate bonds [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 71,126 | 82,824
Gross Unrealized Gains | 965 | 1,565
Gross Unrealized Losses | (19) | (12)
Fair Value | 72,072 | 84,377
Commercial mortgage-backed securities [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 51,414 | 51,529
Gross Unrealized Gains | 3,861 | 6,433
Gross Unrealized Losses |  |
Fair Value | 55,275 | 57,962
U.S. agency-based mortgage-backed securities [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 23,876 | 32,984
Gross Unrealized Gains | 1,966 | 3,063
Gross Unrealized Losses |  | (5)
Fair Value | 25,842 | 36,042
U.S. Treasury securities and obligations of U.S. government agencies [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 11,522 | 11,034
Gross Unrealized Gains | 1,130 | 1,721
Gross Unrealized Losses |  |
Fair Value | 12,652 | 12,755
Asset-backed securities [Member] |  |
Investment [Line Items] |  |
Amortized Cost | 3,521 | 4,229
Gross Unrealized Gains | 216 | 192
Gross Unrealized Losses | (171) | (176)
Fair Value | $ 3,566 | $ 4,245

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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

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- 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
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- 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_
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Period Type: | instant