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Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Fixed-Maturity Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Investments Debt And Equity Securities [Abstract] |  |
Amortized Cost, 2018 | $ 3,167 |
Total fixed-maturity securities, Amortized Cost | 3,167 | $ 3,674
Estimated Fair Value, 2018 | 3,153 |
Total fixed-maturity securities, Estimated Fair Value | $ 3,153 | $ 3,660

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