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