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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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