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Fair Value Measurements - Schedule of Assets and Liabilities Measured at Amortized Cost (Detail) - USD ($) $ in Thousands | Sep. 30, 2016 | Dec. 31, 2015
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Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | $ 620,781 | $ 662,276
States and Political Subdivisions [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 433,734 | 423,754
Corporate Bonds [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 159,659 | 170,573
Commercial Mortgage-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 290 | 37,683
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 12,136 | 14,471
US Treasury Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 7,029 | 7,599
US Government-sponsored Enterprises Debt Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 5,819 | 5,781
Asset-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 2,114 | 2,415
Fair Value, Inputs, Level 1 [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 7,029 | 7,599
Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 7,029 | 7,599
Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 613,752 | 654,677
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 | 433,734 | 423,754
Fair Value, Inputs, Level 2 [Member] | Corporate Bonds [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 159,659 | 170,573
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 | 290 | 37,683
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 | 12,136 | 14,471
Fair Value, Inputs, Level 2 [Member] | US Government-sponsored Enterprises Debt Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 5,819 | 5,781
Fair Value, Inputs, Level 2 [Member] | Asset-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | $ 2,114 | $ 2,415

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