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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Jan. 01, 2020 | Jan. 01, 2019
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of comparable consolidated amounts |  |  |  |  |  |  |  |  |  | 95.00% | 95.00% | 95.00% |  |
Decrease in net income |  |  |  |  |  |  |  |  |  | $ 86,602,000 | $ 92,690,000 | $ 71,632,000 |  |
Decrease to diluted earnings per common share | $ 1.47 | $ 1.21 | $ 1.24 | $ 0.56 | $ 1.76 | $ 1.11 | $ 0.93 | $ 1.01 |  | $ 4.47 | $ 4.80 | $ 3.71 |  |
Right-of-use assets | $ 314,000 |  |  |  | $ 430,000 |  |  |  |  | $ 314,000 | $ 430,000 |  |  |
Operating lease liabilities | 314,000 |  |  |  | 430,000 |  |  |  |  | 314,000 | 430,000 |  |  |
Net decrease to retained earnings, cumulative effect | (224,645,000) |  |  |  | (227,165,000) |  |  |  |  | (224,645,000) | (227,165,000) |  |  |
Credit allowance for held-to-maturity fixed income securities | 274,000 |  |  |  | 0 |  |  |  |  | 274,000 | 0 |  |  |
Credit allowance for reinsurance recoverables | 452,000 |  |  |  | 0 |  |  |  |  | 452,000 | 0 |  |  |
Available-for-sale securities, credit allowance | $ 0 |  |  |  | 0 |  |  |  |  | $ 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 |  |  |  |  |  |  |  |  | 90 days |  |  |  |
Original maturity of cash equivalent |  |  |  |  |  |  |  |  |  | 90 days |  |  |  |
Minimum period of underwriting workers’ compensation insurance from which historical loss data utilize |  |  |  |  |  |  |  |  |  | 35 years |  |  |  |
Experience rated commission and other operating costs |  |  |  |  |  |  |  |  |  | $ 0 | 200,000 | $ 200,000 |  |
Commutation payment adjusted through offset against balance of funds withheld and recoverable from reinsurers accounts |  |  |  |  | $ 8,500,000 |  |  |  |  |  | 8,500,000 |  |  |
Advertising expense |  |  |  |  |  |  |  |  |  | $ 200,000 | $ 400,000 | $ 400,000 |  |
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 2016-02 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Right-of-use assets |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 400,000
Operating lease liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 300,000
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 |
ASU 2016-01 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Decrease in net income |  |  |  |  |  |  |  |  | $ 390,000 |  |  |  |  |
ASU 2016-01 [Member] | Common Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Decrease to diluted earnings per common share |  |  |  |  |  |  |  |  | $ 0.02 |  |  |  |  |

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