Amount, before investment expense, of income earned from investments in securities, real estate, and alternate investments. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

No definition available.

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- Definition

Amount of expenses related to the generation of investment income.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.2)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.9)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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- Definition

Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 16

-Article 12

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.2)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 2

-Article 7

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Investment - Major Categories of the Company's Net Investment Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investment [Line Items]
Total gross investment income									$ 28,891amsf_GrossInvestmentIncome	$ 28,402amsf_GrossInvestmentIncome	$ 28,102amsf_GrossInvestmentIncome
Investment expenses									(1,677)	(1,373)	(1,084)
Net investment income	7,166us-gaap_NetInvestmentIncome	6,495us-gaap_NetInvestmentIncome	6,845us-gaap_NetInvestmentIncome	6,708us-gaap_NetInvestmentIncome	6,763us-gaap_NetInvestmentIncome	6,947us-gaap_NetInvestmentIncome	6,649us-gaap_NetInvestmentIncome	6,670us-gaap_NetInvestmentIncome	27,214us-gaap_NetInvestmentIncome	27,029us-gaap_NetInvestmentIncome	27,018us-gaap_NetInvestmentIncome
Fixed Maturity Securities Available for Sale [Member]
Investment [Line Items]
Total gross investment income									26,622	26,422	27,167
Short-term Investments [Member]
Investment [Line Items]
Total gross investment income									426	634	489
Equity Securities [Member]
Investment [Line Items]
Total gross investment income									232	543	446
Other Investments [Member]
Investment [Line Items]
Total gross investment income									$ 1,611	$ 803