Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	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