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Investments - Additional Information (Detail) | 12 Months Ended
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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 |

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