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Investments - Gross Unrealized Gains and Losses and Cost and Fair Value of Investments Classified as Held-to-maturity (Detail) (USD $) In Thousands, unless otherwise specified | Jun. 30, 2013 | Dec. 31, 2012
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Investment [Line Items] |  |
Cost or Amortized Cost | $ 565,874 | $ 583,287
Gross Unrealized Gains | 30,173 | 44,361
Gross Unrealized Losses | (1,516) | (299)
Fair Value | 594,531 | 627,349
States and political subdivisions [Member] |  |
Investment [Line Items] |  |
Cost or Amortized Cost | 395,641 | 400,687
Gross Unrealized Gains | 21,298 | 31,387
Gross Unrealized Losses | (1,262) | (106)
Fair Value | 415,677 | 431,968
Corporate bonds [Member] |  |
Investment [Line Items] |  |
Cost or Amortized Cost | 77,235 | 82,824
Gross Unrealized Gains | 1,045 | 1,565
Gross Unrealized Losses | (74) | (12)
Fair Value | 78,206 | 84,377
Commercial mortgage-backed securities [Member] |  |
Investment [Line Items] |  |
Cost or Amortized Cost | 51,502 | 51,529
Gross Unrealized Gains | 4,230 | 6,433
Fair Value | 55,732 | 57,962
U.S. agency-based mortgage-backed securities [Member] |  |
Investment [Line Items] |  |
Cost or Amortized Cost | 26,732 | 32,984
Gross Unrealized Gains | 2,179 | 3,063
Gross Unrealized Losses |  | (5)
Fair Value | 28,911 | 36,042
U.S. Treasury securities and obligations of U.S. government agencies [Member] |  |
Investment [Line Items] |  |
Cost or Amortized Cost | 11,020 | 11,034
Gross Unrealized Gains | 1,205 | 1,721
Fair Value | 12,225 | 12,755
Asset-backed securities [Member] |  |
Investment [Line Items] |  |
Cost or Amortized Cost | 3,744 | 4,229
Gross Unrealized Gains | 216 | 192
Gross Unrealized Losses | (180) | (176)
Fair Value | $ 3,780 | $ 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
Name: | us-gaap_HeldToMaturitySecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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
Name: | us-gaap_HeldToMaturitySecuritiesFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant