Fair Value Measurements - Summary of Carrying Amounts and Corresponding Fair Values for Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities—held-to-maturity	$ 603,720	$ 621,654
Fixed maturity securities—available-for-sale	411,904	414,279
Equity securities	50,843	43,437
Short-term investments	87,279	45,898
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities—held-to-maturity	575,108	585,130
Fixed maturity securities—available-for-sale	411,904	414,279
Equity securities	50,843	43,437
Short-term investments	87,279	45,898
Cash and cash equivalents	33,074	61,757
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities—held-to-maturity	603,720	621,654
Fixed maturity securities—available-for-sale	411,904	414,279
Equity securities	50,843	43,437
Short-term investments	87,279	45,898
Cash and cash equivalents	$ 33,074	$ 61,757