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Fair Value Measurements - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Sep. 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 $473,315 and $461,236 in 2018 and 2017, respectively) | $ 468,019 | $ 465,594
Equity securities, Fair Value | 14,791 | 9,282
Total |  | 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 $473,315 and $461,236 in 2018 and 2017, respectively) | 468,019 | 465,594
Total | 482,810 | 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 $473,315 and $461,236 in 2018 and 2017, respectively) | 64,804 | 66,218
Total | 79,593 | 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 $473,315 and $461,236 in 2018 and 2017, respectively) | 403,215 | 399,376
Total | 403,215 | 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 | 2 | 34
Fair Value, Measurements, Recurring [Member] | Common Stock [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities, Fair Value | 14,791 | 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] |  |
Equity securities, Fair Value | 14,789 | 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] |  |
Equity securities, 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 $473,315 and $461,236 in 2018 and 2017, respectively) | 227,487 | 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 $473,315 and $461,236 in 2018 and 2017, respectively) | 227,487 | 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 $473,315 and $461,236 in 2018 and 2017, respectively) | 162,948 | 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 $473,315 and $461,236 in 2018 and 2017, respectively) | 162,948 | 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 $473,315 and $461,236 in 2018 and 2017, respectively) | 12,780 | 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 $473,315 and $461,236 in 2018 and 2017, respectively) | 12,780 | 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 $473,315 and $461,236 in 2018 and 2017, respectively) | 64,804 | 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 $473,315 and $461,236 in 2018 and 2017, respectively) | $ 64,804 | $ 66,218

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