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Investments - Additional Information (Detail) $ in Thousands | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2021 USD ($) Security | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) Security | Sep. 30, 2020 USD ($) | Jun. 30, 2021 USD ($) | Dec. 31, 2020 USD ($)
Marketable Securities [Line Items] |  |  |  |  |  |
Individual fixed maturity held in unrealized loss position | Security | 22 |  | 22 |  |  |
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 | 378 |  | 378 |  |  |
Fixed maturity securities, allowance for credit loss | $ 172 |  | $ 172 |  | $ 157 | $ 274
Available-for-sale, Allowance for Credit Losses | 0 |  | 0 |  |  | 0
Net realized gains (losses) |  | $ 300 | 1,500 | $ 1,500 |  |
Net unrealized gains (losses) on equity securities | (771) | $ 844 | 8,026 | $ (2,349) |  |
Corporate Bonds [Member] |  |  |  |  |  |
Marketable Securities [Line Items] |  |  |  |  |  |
Fixed maturity securities, allowance for credit loss | $ 123 |  | $ 123 |  | 111 | 225
Number of majority securities | Security | 20 |  | 20 |  |  |
States and Political Subdivisions [Member] |  |  |  |  |  |
Marketable Securities [Line Items] |  |  |  |  |  |
Fixed maturity securities, allowance for credit loss | $ 45 |  | $ 45 |  | $ 41 | $ 42
Number of majority securities | Security | 355 |  | 355 |  |  |
Corporate Fixed Income Securities [Member] | Maximum [Member] |  |  |  |  |  |
Marketable Securities [Line Items] |  |  |  |  |  |
Investments maturity period using default rate |  |  | 20 years |  |  |

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