Expected Cash Dividend Per Share

No definition available.

Name:	amsf_ExpectedCashDividendPerShare
Namespace Prefix:	amsf_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- 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

- Definition

The per share amount of a dividend declared, but not paid, as of the financial reporting date.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- 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

- 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

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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
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- Details

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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
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