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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 |  |

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