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Investments - Additional Information (Detail)
 3 Months Ended 9 Months Ended
Sep. 30, 2020
USD ($)
Security
Sep. 30, 2019
USD ($)
Sep. 30, 2020
USD ($)
Security
Sep. 30, 2019
USD ($)
Dec. 31, 2019
USD ($)
Marketable Securities [Line Items]          
Individual fixed maturity held in unrealized loss position | Security 8   8    
Individual fixed maturity held in unrealized loss position longer than 12 months | Security 0   0    
Number of fixed maturity securities, allowance for credit loss | Security 400   400    
Fixed maturity securities, allowance for credit loss $ 288,000   $ 288,000   $ 0
Fixed maturity securities, available-for-sale, allowance for credit losses 0   0   $ 0
Net realized gains (losses) 300,000   1,500,000    
Net unrealized gains (losses) on equity securities 844,000 $ 365,000 (2,349,000) $ 3,165,000  
Corporate Bonds [Member]          
Marketable Securities [Line Items]          
Fixed maturity securities, allowance for credit loss $ 242,000   $ 242,000    
Number of majority securities | Security 28   28    
States and Political Subdivisions [Member]          
Marketable Securities [Line Items]          
Fixed maturity securities, allowance for credit loss $ 38,000   $ 38,000    
Number of majority securities | Security 369   369    
Corporate Fixed Income Securities [Member] | Maximum [Member]          
Marketable Securities [Line Items]          
Investments maturity period using default rate     20 years