XML 74 R58.htm IDEA: XBRL DOCUMENT
v3.3.1.900

Premiums Receivable - Summarizes the Activity in the Allowance for Doubtful Accounts (Parenthetical) (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Receivables [Line Items] |  |  |
Provision for bad debts | $ 806 | $ 673 | $ 52
One-Time Adjustment [Member] |  |  |
Receivables [Line Items] |  |  |
Provision for bad debts |  |  | $ 1,700

X
- Definition Receivables [Line Items] + References No definition available. + Details
Name: | amsf_ReceivablesLineItems
Namespace Prefix: | amsf_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- 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 + Details
Name: | us-gaap_PremiumsReceivableAllowanceForDoubtfulAccountsProvisionChargedToExpense1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=amsf_OneTimeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |