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Premiums Receivable - Summarizes the Activity in the Allowance for Doubtful Accounts (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Receivables [Line Items]
Provision for bad debts	$ 673us-gaap_PremiumsReceivableAllowanceForDoubtfulAccountsProvisionChargedToExpense1	$ 52us-gaap_PremiumsReceivableAllowanceForDoubtfulAccountsProvisionChargedToExpense1	$ 2,230us-gaap_PremiumsReceivableAllowanceForDoubtfulAccountsProvisionChargedToExpense1
One-Time Adjustment [Member]
Receivables [Line Items]
Provision for bad debts		$ 1,700us-gaap_PremiumsReceivableAllowanceForDoubtfulAccountsProvisionChargedToExpense1 / us-gaap_StatementEquityComponentsAxis = amsf_OneTimeMember

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- Definition

Amount of expense for known and estimated loss on collection of premiums receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 4

-Subparagraph (SX 210.12-09)

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 09

-Article 12

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Data Type:	xbrli:monetaryItemType
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