Fair Value Measurements - Summary of Carrying Amounts and Corresponding Fair Values for Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities—held-to-maturity	$ 569,445	$ 621,654
Fixed maturity securities—available-for-sale	341,838	414,279
Equity securities	56,351	43,437
Short-term investments	94,774	45,898
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities—held-to-maturity	541,224	585,130
Fixed maturity securities—available-for-sale	341,838	414,279
Equity securities	56,351	43,437
Short-term investments	94,774	45,898
Cash and cash equivalents	123,128	61,757
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities—held-to-maturity	569,445	621,654
Fixed maturity securities—available-for-sale	341,838	414,279
Equity securities	56,351	43,437
Short-term investments	94,774	45,898
Cash and cash equivalents	$ 123,128	$ 61,757