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Basis of Presentation - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Jan. 01, 2020	Dec. 31, 2019
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Percentage of comparable consolidated amounts	95.00%	95.00%
Net decrease to retained earnings, cumulative effect	$ (269,022,000)			$ (227,165,000)
Credit allowance for held-to-maturity fixed income securities	288,000			0
Credit allowance for reinsurance recoverables	379,000			0
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
Minimum [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Trading price of securities excluded from watch list	$ 95
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
Cumulative Effect, Period of Adoption, Adjustment [Member] | ASU 2016-13 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Net decrease to retained earnings, cumulative effect			$ 594,000

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