Investments - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) Security	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Marketable Securities [Line Items]
Individual fixed maturity held in unrealized loss position | Security	29
Individual fixed maturity held in unrealized loss position longer than 12 months | Security	0
Number of fixed maturity securities, allowance for credit loss | Security	410
Fixed maturity securities, allowance for credit loss	$ 166		$ 274
Available-for-sale, Allowance for Credit Losses	0		0
Net realized gains	300	$ 1,000
Net unrealized gains (losses) on equity securities	5,511	$ (8,763)
Corporate Bonds [Member]
Marketable Securities [Line Items]
Fixed maturity securities, allowance for credit loss	$ 116		225
Number of majority securities | Security	27
States and Political Subdivisions [Member]
Marketable Securities [Line Items]
Fixed maturity securities, allowance for credit loss	$ 44		$ 42
Number of majority securities | Security	380
Corporate Fixed Income Securities [Member] | Maximum [Member]
Marketable Securities [Line Items]
Investments maturity period using default rate	20 years