Reinsurance - Schedule of the Effect of Reinsurance on Premiums Written and Earned (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Premiums Written and Earned [Abstract]
Gross Written	$ 393,819us-gaap_PremiumsWrittenGross	$ 372,177us-gaap_PremiumsWrittenGross	$ 328,823us-gaap_PremiumsWrittenGross
Ceded Written	(13,793)us-gaap_CededPremiumsWritten	(18,425)us-gaap_CededPremiumsWritten	(16,305)us-gaap_CededPremiumsWritten
Net premiums Written	380,026us-gaap_PremiumsWrittenNet	353,752us-gaap_PremiumsWrittenNet	312,518us-gaap_PremiumsWrittenNet
Gross Earned	389,540amsf_PremiumsEarnedGross	348,408amsf_PremiumsEarnedGross	306,994amsf_PremiumsEarnedGross
Ceded Earned	(13,793)us-gaap_CededPremiumsEarned	(18,425)us-gaap_CededPremiumsEarned	(16,305)us-gaap_CededPremiumsEarned
Net premiums Earned	$ 375,747	$ 329,983	$ 290,689

Amount, before premiums ceded to other entities of premiums earned.

No definition available.

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- Definition

Amount of earned premiums ceded to other entities.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 235

-Section S99

-Paragraph 2

-Subparagraph (SX 210.12-17)

-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.1)

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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-Subparagraph (b),(c)(2)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1

-Article 7

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

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Balance Type:	debit
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- Definition

Amount of written premiums ceded to other entities.

+ References

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- Definition

Amount, after premiums ceded to other entities and premiums assumed by the entity, of property and casualty premiums earned.

+ 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 210.7-04.1)

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1

-Article 7

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6486662&loc=d3e27175-158546

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

-Publisher FASB

-Name Accounting Standards Codification

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-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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- Definition

Amount of direct and assumed premiums written.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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- Definition

Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written.

+ References

+ Details

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