XML 48 R38.htm IDEA: XBRL DOCUMENT
v3.7.0.1

Comprehensive Income and Accumulated Other Comprehensive Income - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract] |  |  |  |
Comprehensive income | $ 17,437 | $ 19,568 | $ 31,913 | $ 46,618
Comprehensive income tax rate |  |  | 35.00% |

X
- Definition Other comprehensive income loss available for sale securities tax rate. + References No definition available. + Details
Name: | amsf_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesTaxRate
Namespace Prefix: | amsf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTaxAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e557-108580 + Details
Name: | us-gaap_ComprehensiveIncomeNetOfTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration