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		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) for changes in fair value and net settlements of derivatives	$ (2,207)	$ (4,666)	$ (22,319)	$ (7,233)
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) for changes in fair value and net settlements of derivatives	1,100	200	1,700	400
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	(1,449)	(4,666)	(10,872)	(7,233)
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	(59)	0	(2,458)	0
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	$ (699)	$ 0	$ (8,989)	$ 0