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Summary of Net Losses Included in Consolidated Statement 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, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Jun. 30, 2014
Interest Rate Swap | Net Realized Investment Gains |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Net gain (loss) for changes in fair value and net settlements of derivatives | $ 5,057 | $ (6,018) | $ (825) | $ (12,360)

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