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Investment Securities - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Estimated Fair Values of Fixed-Maturity Securities (Detail) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Total fixed-maturity securities, Amortized Cost | $ 3,674 |
Total fixed-maturity securities, Gross Unrealized Losses | (14) |
Total fixed-maturity securities, Estimated Fair Value | 3,660 |
Unrestricted Fixed-Maturity Securities [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Total fixed-maturity securities, Amortized Cost | 2,151 |
Total fixed-maturity securities, Gross Unrealized Losses | (9) |
Total fixed-maturity securities, Estimated Fair Value | 2,142 | [1]
Fixed-Maturity Securities Held in Trust [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Total fixed-maturity securities, Amortized Cost | 1,523 |
Total fixed-maturity securities, Gross Unrealized Losses | (5) |
Total fixed-maturity securities, Estimated Fair Value | 1,518 | [1]
Obligations of U.S. Government Sponsored Enterprises [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Total fixed-maturity securities, Amortized Cost | 2,249 |
Total fixed-maturity securities, Gross Unrealized Losses | (10) |
Total fixed-maturity securities, Estimated Fair Value | 2,239 |
Corporate Debt Securities [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Total fixed-maturity securities, Amortized Cost | 1,425 |
Total fixed-maturity securities, Gross Unrealized Losses | (4) |
Total fixed-maturity securities, Estimated Fair Value | $ 1,421 |
[1] | Balances derived from audited financial statements as of December 31, 2017.

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