Fair Value Measurements - Summary of Carrying Amounts and Corresponding Fair Values for Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities—held-to-maturity	$ 607,339	$ 621,343
Fixed maturity securities—available-for-sale	426,237	441,146
Equity securities	35,822	27,903
Short-term investments	90,826	56,548
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities—held-to-maturity	572,508	599,421
Fixed maturity securities—available-for-sale	426,237	441,146
Equity securities	35,822	27,903
Short-term investments	90,826	56,548
Cash and cash equivalents	88,168	43,813
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity securities—held-to-maturity	607,339	621,343
Fixed maturity securities—available-for-sale	426,237	441,146
Equity securities	35,822	27,903
Short-term investments	90,826	56,548
Cash and cash equivalents	$ 88,168	$ 43,813