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Fair Values of Financial Instruments - Schedule of Assets and Liabilities Measured at Amortized Cost (Detail) - USD ($) $ in Thousands | Dec. 31, 2015 | Dec. 31, 2014
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Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | $ 662,276 | $ 664,371
States and Political Subdivisions [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 423,754 | 405,665
Corporate Bonds [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 170,573 | 176,734
Commercial Mortgage-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 37,683 | 48,529
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 14,471 | 18,672
US Treasury Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 13,380 | 11,792
Asset-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 2,415 | 2,979
Fair Value, Inputs, Level 1 [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 13,380 | 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 | 13,380 | 11,792
Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 648,896 | 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 | 423,754 | 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 | 170,573 | 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 | 37,683 | 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 | 14,471 | 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 | $ 2,415 | $ 2,979

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- Definition 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 + Details
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