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Fair Value Measurements - Schedule of Assets and Liabilities Measured at Amortized Cost (Detail) - USD ($) $ in Thousands | Mar. 31, 2017 | Dec. 31, 2016
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Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | $ 603,295 | $ 568,931
States and Political Subdivisions [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 435,792 | 399,483
Corporate Bonds [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 140,737 | 144,016
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 10,024 | 10,915
U.S. Treasury Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 6,780 | 6,779
US Government-sponsored Enterprises Debt Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 8,016 | 5,637
Asset-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 1,946 | 2,031
Commercial Mortgage-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value |  | 70
Fair Value, Inputs, Level 1 [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 6,780 | 6,779
Fair Value, Inputs, Level 1 [Member] | U.S. Treasury Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 6,780 | 6,779
Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 596,515 | 562,152
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 | 435,792 | 399,483
Fair Value, Inputs, Level 2 [Member] | Corporate Bonds [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | 140,737 | 144,016
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 | 10,024 | 10,915
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 | 8,016 | 5,637
Fair Value, Inputs, Level 2 [Member] | Asset-Backed Securities [Member] |  |
Fair Value Measurements Disclosure [Line Items] |  |
Securities held-to-maturity-fixed maturity, Fair Value | $ 1,946 | 2,031
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 |  | $ 70

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