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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
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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