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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |
Percentage of comparable consolidated amounts | 95.00% | 95.00% | 95.00%
Right-of-use assets | $ 29,000 | $ 106,000 |
Operating lease liabilities | 29,000 | 106,000 |
Net decrease to retained earnings, cumulative effect | (114,289,000) | (145,512,000) |
Credit allowance for held-to-maturity fixed income securities | 182,000 | 239,000 |
Credit allowance for reinsurance recoverables | 360,000 | 372,000 |
Available-for-sale securities, credit allowance | $ 0 | 0 |
Original maturity of cash equivalent | 90 days |  |
Minimum period of underwriting workers? compensation insurance from which historical loss data utilize | 38 years |  |
Experience rated commission and other operating costs | $ 1,700,000 | 100,000 | $ 1,000,000
Advertising expense | $ 200,000 | $ 200,000 | $ 200,000
Assessment Related premium Payment period | 1 year |  |
Assessment Related loss Payment period | 1 year |  |
Buildings [Member] |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |
Estimated useful life of assets | 39 years |  |
Maximum [Member] |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |
Short-term investments maturity period | 1 year |  |
Maximum [Member] | Other Fixed Assets [Member] |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |
Estimated useful life of assets | 7 years |  |
Minimum [Member] |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |
Short-term investments maturity period | 90 days |  |
Minimum [Member] | Other Fixed Assets [Member] |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |
Estimated useful life of assets | 3 years |  |
Corporate Fixed Income Securities [Member] | Maximum [Member] |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |
Investments maturity period using default rate | 20 years |  |

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