Investments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020 USD ($) Security	Dec. 31, 2019 USD ($)
Marketable Securities [Line Items]
Individual fixed maturity held in unrealized loss position | Security	2
Individual fixed maturity held in unrealized loss position longer than 12 months | Security	12
Number of fixed maturity securities, allowance for credit loss | Security	424
Fixed maturity securities, allowance for credit loss	$ 274,000	$ 0
Impairment losses recognized		0
Held-to-Maturity Securities [Member]
Marketable Securities [Line Items]
Investments on deposit with regulatory agencies	19,900,000
Corporate Bonds [Member]
Marketable Securities [Line Items]
Short-term investments	6,100,000
Fixed maturity securities, allowance for credit loss	$ 225,000
Number of majority securities | Security	393
States and Political Subdivisions [Member]
Marketable Securities [Line Items]
Short-term investments	$ 9,100,000	1,900,000
Fixed maturity securities, allowance for credit loss	$ 42,000
Number of majority securities | Security	28
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Marketable Securities [Line Items]
Short-term investments	$ 30,700,000	$ 54,600,000
Corporate Fixed Income Securities [Member] | Maximum [Member]
Marketable Securities [Line Items]
Investments maturity period using default rate	20 years