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Fair Value Measurements - Schedule of Assets and Liabilities Measured at Amortized Cost (Detail) (USD $) In Thousands, unless otherwise specified | Jun. 30, 2014 | Dec. 31, 2013
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Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | $ 607,512 | $ 561,179
States and political subdivisions [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 414,136 | 399,155
Corporate bonds [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 103,624 | 68,243
Commercial mortgage-backed securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 52,094 | 54,244
U.S. agency-based mortgage-backed securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 21,062 | 23,565
US Treasury securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 7,339 | 6,602
Obligations of U.S. government agencies [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 6,051 | 5,914
Asset-backed securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 3,206 | 3,456
Fair Value, Inputs, Level 1 [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 7,339 | 6,602
Fair Value, Inputs, Level 1 [Member] | US Treasury securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 7,339 | 6,602
Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 600,173 | 554,577
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 | 414,136 | 399,155
Fair Value, Inputs, Level 2 [Member] | Corporate bonds [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 103,624 | 68,243
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 | 52,094 | 54,244
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 | 21,062 | 23,565
Fair Value, Inputs, Level 2 [Member] | Obligations of U.S. government agencies [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 6,051 | 5,914
Fair Value, Inputs, Level 2 [Member] | Asset-backed securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | $ 3,206 | $ 3,456

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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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