XML 115 R102.htm IDEA: XBRL DOCUMENT
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Subsequent Events - Additional Information (Detail) - $ / shares | Feb. 22, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
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Subsequent Event [Line Items] |  |  |  |  |  |  |
Cash dividend per share |  |  | $ 0.20 | $ 0.18 | $ 0.15 | $ 0.12 | $ 0.08
Scenario, Forecast [Member] |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Expected cash dividend per share |  | $ 0.88 |  |  |  |  |
Dividend Declared [Member] |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Cash dividend per share |  |  | $ 0.20 |  |  |  |
Subsequent Event [Member] | Dividend Declared [Member] |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |
Cash dividend per share | $ 0.22 |  |  |  |  |  |
Date of payment to shareholders entitled to dividends | Mar. 23, 2018 |  |  |  |  |  |
Date of record of shareholders entitled to dividends | Mar. 09, 2018 |  |  |  |  |  |

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- Definition Expected Cash Dividend Per Share + References No definition available. + Details
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Namespace Prefix: | amsf_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

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- Definition Date the declared dividend will be paid, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition The per share amount of a dividend declared, but not paid, as of the financial reporting date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 + Details
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Balance Type: | na
Period Type: | instant

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- Definition Date the holder must own the stock to be entitled to the dividend, in CCYY-MM-DD format. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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