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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) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2017 | Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Total fixed-maturity securities, Amortized Cost | $ 10,432 | $ 76,912 |
Total fixed-maturity securities, Gross Unrealized Gains |  | 15 |
Total fixed-maturity securities, Gross Unrealized Losses | (10) | (96) |
Total fixed-maturity securities, Estimated Fair Value | 10,422 | 76,831 |
Unrestricted Fixed-Maturity Securities [Member] |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Total fixed-maturity securities, Amortized Cost | 1,402 | 47,635 |
Total fixed-maturity securities, Gross Unrealized Losses | (2) | (10) |
Total fixed-maturity securities, Estimated Fair Value | 1,400 | 47,625 | [1]
Fixed-Maturity Securities Held in Trust [Member] |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Total fixed-maturity securities, Amortized Cost | 9,030 | 29,277 |
Total fixed-maturity securities, Gross Unrealized Gains |  | 15 |
Total fixed-maturity securities, Gross Unrealized Losses | (8) | (86) |
Total fixed-maturity securities, Estimated Fair Value | 9,022 | 29,206 | [1]
U.S. Government Treasury Securities [Member] |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Total fixed-maturity securities, Amortized Cost | 250 | 249 |
Total fixed-maturity securities, Estimated Fair Value | 250 | 249 |
Obligations of U.S. Government Sponsored Enterprises [Member] |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Total fixed-maturity securities, Amortized Cost | 2,749 | 59,450 |
Total fixed-maturity securities, Gross Unrealized Gains |  | 1 |
Total fixed-maturity securities, Gross Unrealized Losses | (7) | (80) |
Total fixed-maturity securities, Estimated Fair Value | 2,742 | 59,371 |
Corporate Debt Securities [Member] |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Total fixed-maturity securities, Amortized Cost | 3,683 | 11,415 |
Total fixed-maturity securities, Gross Unrealized Gains |  | 9 |
Total fixed-maturity securities, Gross Unrealized Losses | (2) | (9) |
Total fixed-maturity securities, Estimated Fair Value | 3,681 | 11,415 |
Foreign Corporate Debt Securities [Member] |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Total fixed-maturity securities, Amortized Cost | 3,750 | 5,798 |
Total fixed-maturity securities, Gross Unrealized Gains |  | 5 |
Total fixed-maturity securities, Gross Unrealized Losses | (1) | (7) |
Total fixed-maturity securities, Estimated Fair Value | $ 3,749 | $ 5,796 |
[1] | Balances derived from audited financial statements as of December 31, 2016.

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