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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Jan. 01, 2020
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 | $ 106,000 | $ 193,000 |  |
Operating lease liabilities | 106,000 | 193,000 |  |
Net decrease to retained earnings, cumulative effect | (145,512,000) | (190,492,000) |  |
Credit allowance for held-to-maturity fixed income securities | 239,000 | 195,000 |  |
Credit allowance for reinsurance recoverables | 372,000 | 440,000 |  |
Available-for-sale securities, credit allowance | $ 0 | 0 |  |
Trading securities, par value per share | $ 100 |  |  |
Reinsurance recoverables, current credit rating period | 10 years |  |  |
Reinsurance treaty term to be commuted by end of current credit rating period | 3 years |  |  |
Other-than-temporary impairment loss, debt securities, portion recognized in earnings | $ 0 |  |  |
Financial assets purchased with credit deterioration, financing receivable | 0 |  |  |
Financial assets purchased with credit deterioration, held-to-maturity debt securities | $ 0 |  |  |
Financial assets purchased with credit deterioration, available-for-sale debt securities |  | 0 |  |
Threshold period for writing off balances receivable | 90 days |  |  |
Original maturity of cash equivalent | 90 days |  |  |
Minimum period of underwriting workers? compensation insurance from which historical loss data utilize | 37 years |  |  |
Experience rated commission and other operating costs | $ 100,000 | 1,000,000.0 | $ 0 |
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] |  |  |  |
Trading price of securities excluded from watch list | $ 95 |  |  |
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 |  |  |
Municipal Fixed Income Securities [Member] | Maximum [Member] |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |
Investments maturity period using default rate | 20 years |  |  |
ASU 2016-13 [Member] |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |
Credit allowance for held-to-maturity fixed income securities |  |  |  | $ 243,000
Credit allowance for reinsurance recoverables |  |  |  | 351,000
ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member] |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |
Net decrease to retained earnings, cumulative effect |  |  |  | $ 594,000

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