Fair Value Measurements - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 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 $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