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Fair Value Measurements - Schedule of Assets and Liabilities Measured at Amortized Cost (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	$ 679,893	$ 664,371
States and Political Subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	433,916	405,665
Corporate Bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	170,490	176,734
Commercial Mortgage-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	43,982	48,529
U.S. Agency-Based Mortgage-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	15,486	18,672
US Treasury Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	13,439	11,792
Asset-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	2,580	2,979
Fair Value, Inputs, Level 1 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	13,439	11,792
Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	13,439	11,792
Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	666,454	652,579
Fair Value, Inputs, Level 2 [Member] | States and Political Subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	433,916	405,665
Fair Value, Inputs, Level 2 [Member] | Corporate Bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	170,490	176,734
Fair Value, Inputs, Level 2 [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	43,982	48,529
Fair Value, Inputs, Level 2 [Member] | U.S. Agency-Based Mortgage-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	15,486	18,672
Fair Value, Inputs, Level 2 [Member] | Asset-Backed Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	$ 2,580	$ 2,979

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Fair Value Measurements Disclosure [Line Items]

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Fair value of investments in debt securities classified as held-to-maturity.

+ References

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



-Publisher FASB



-Name Accounting Standards Codification



-Topic 320



-SubTopic 10



-Section 50



-Paragraph 5



-Subparagraph (aa)



-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563



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



-Publisher FASB



-Name Accounting Standards Codification



-Topic 320



-SubTopic 10



-Section 50



-Paragraph 5



-Subparagraph (f)



-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563



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



-Publisher FASB



-Name Accounting Standards Codification



-Topic 320



-SubTopic 10



-Section 50



-Paragraph 5



-Subparagraph (a)



-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

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