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Summary of Net Gain (Loss) Included in Consolidated Statements of Operations for Changes in Fair Value of Derivatives and Periodic Net Interest Settlements Under Derivatives (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) for changes in fair value and net settlements of derivatives	$ 160	$ (2,171)	$ (22,159)	$ (9,404)
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) for changes in fair value and net settlements of derivatives	1,400	300	3,100	700
Interest Rate Swap | Net Realized Investment Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) for changes in fair value and net settlements of derivatives	45	(1,831)	(10,827)	(9,064)
Futures Contracts on Bonds | Net Realized Investment Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) for changes in fair value and net settlements of derivatives	115	15	(2,343)	15
Futures Contracts on Equities | Net Realized Investment Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) for changes in fair value and net settlements of derivatives	$ 0	$ (355)	$ (8,989)	$ (355)

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Amount of gain (loss) included in earnings for the period from the increase (decrease) in fair value of interest rate derivatives not designated as hedging instruments.

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