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Fair Value Measurements - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturity securities—available-for-sale, at fair value (cost $443,358 and $461,236 in 2018 and 2017, respectively) | $ 440,762 | $ 465,594
Available-for-sale—equity, Fair Value | 13,590 | 9,282
Total available-for-sale | 454,352 | 474,876
Fair Value, Measurements, Recurring [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturity securities—available-for-sale, at fair value (cost $443,358 and $461,236 in 2018 and 2017, respectively) | 440,762 | 465,594
Total available-for-sale | 454,352 | 474,876
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturity securities—available-for-sale, at fair value (cost $443,358 and $461,236 in 2018 and 2017, respectively) | 65,141 | 66,218
Total available-for-sale | 78,729 | 75,466
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturity securities—available-for-sale, at fair value (cost $443,358 and $461,236 in 2018 and 2017, respectively) | 375,621 | 399,376
Total available-for-sale | 375,621 | 399,376
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total available-for-sale | 2 | 34
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale—equity, Fair Value | 13,590 | 9,282
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale—equity, Fair Value | 13,588 | 9,248
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale—equity, Fair Value | 2 | 34
Fair Value, Measurements, Recurring [Member] | States and Political Subdivisions [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturity securities—available-for-sale, at fair value (cost $443,358 and $461,236 in 2018 and 2017, respectively) | 218,300 | 251,140
Fair Value, Measurements, Recurring [Member] | States and Political Subdivisions [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturity securities—available-for-sale, at fair value (cost $443,358 and $461,236 in 2018 and 2017, respectively) | 218,300 | 251,140
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturity securities—available-for-sale, at fair value (cost $443,358 and $461,236 in 2018 and 2017, respectively) | 143,959 | 130,222
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturity securities—available-for-sale, at fair value (cost $443,358 and $461,236 in 2018 and 2017, respectively) | 143,959 | 130,222
Fair Value, Measurements, Recurring [Member] | U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturity securities—available-for-sale, at fair value (cost $443,358 and $461,236 in 2018 and 2017, respectively) | 13,362 | 18,014
Fair Value, Measurements, Recurring [Member] | U.S. Agency-Based Mortgage-Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturity securities—available-for-sale, at fair value (cost $443,358 and $461,236 in 2018 and 2017, respectively) | 13,362 | 18,014
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturity securities—available-for-sale, at fair value (cost $443,358 and $461,236 in 2018 and 2017, respectively) | 65,141 | 66,218
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturity securities—available-for-sale, at fair value (cost $443,358 and $461,236 in 2018 and 2017, respectively) | $ 65,141 | $ 66,218

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