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Comprehensive Income and Accumulated Other Comprehensive Income - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Comprehensive income	$ 19,248	$ 13,596	$ 47,939	$ 43,174
Comprehensive income tax rate			35.00%

X

- Definition

Other comprehensive income loss available for sale securities tax rate.

+ References

No definition available.

+ Details

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Namespace Prefix:	amsf_
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X

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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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



-Glossary Other Comprehensive Income



-URI http://asc.fasb.org/extlink&oid=51831270



Reference 2: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Glossary Comprehensive Income



-URI http://asc.fasb.org/extlink&oid=51831223



Reference 3: 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=36458714&loc=d3e557-108580



Reference 4: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Glossary Net Income



-URI http://asc.fasb.org/extlink&oid=51831255

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