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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 | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Total fixed-maturity securities, Amortized Cost | $ 3,167 | $ 3,674 |
Total fixed-maturity securities, Gross Unrealized Losses | (14) | (14) |
Total fixed-maturity securities, Estimated Fair Value | 3,153 | 3,660 |
Unrestricted Fixed-Maturity Securities [Member] |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Total fixed-maturity securities, Amortized Cost | 2,917 | 2,151 |
Total fixed-maturity securities, Gross Unrealized Losses | (13) | (9) |
Total fixed-maturity securities, Estimated Fair Value | 2,904 | 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 | 250 | 1,523 |
Total fixed-maturity securities, Gross Unrealized Losses | (1) | (5) |
Total fixed-maturity securities, Estimated Fair Value | 249 | 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 | 2,250 | 2,249 |
Total fixed-maturity securities, Gross Unrealized Losses | (8) | (10) |
Total fixed-maturity securities, Estimated Fair Value | 2,242 | 2,239 |
Corporate Debt Securities [Member] |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Total fixed-maturity securities, Amortized Cost | 917 | 1,425 |
Total fixed-maturity securities, Gross Unrealized Losses | (6) | (4) |
Total fixed-maturity securities, Estimated Fair Value | $ 911 | $ 1,421 |
[1] | Balances derived from audited financial statements as of December 31, 2017.

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- Definition Amount of investments in debt securities classified as trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),1) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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