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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) for changes in fair value and net settlements of derivatives	$ (2,171)	$ 1,311	$ (9,404)	$ 6,959
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) for changes in fair value and net settlements of derivatives	300	400	700	1,500
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	(1,831)	1,311	(9,064)	6,959
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	15	0	15	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	$ (355)	$ 0	$ (355)	$ 0