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Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Investment [Line Items] |  |
Within one year, Amortized Cost | $ 80,180 |
After one year through five years, Amortized Cost | 260,292 |
After five years through ten years, Amortized Cost | 85,673 |
After ten years, Amortized Cost | 167,046 |
Held-to-maturity, Amortized Cost, Totals | 602,830 | $ 629,668
Within one year, Fair Value | 80,293 |
After one year through five years, Fair Value | 259,523 |
After five years through ten years, Fair Value | 84,902 |
After ten years, Fair Value | 165,242 |
Held-to-maturity, Fair Value, Totals | 599,885 | 639,309
U.S. Agency-Based Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost | 8,535 |
Held-to-maturity, Amortized Cost, Totals | 8,535 | 10,260
Held-to-maturity, Fair Value | 8,807 |
Held-to-maturity, Fair Value, Totals | 8,807 | 10,845
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Held-to-maturity, Amortized Cost | 1,104 |
Held-to-maturity, Amortized Cost, Totals | 1,104 | 1,299
Held-to-maturity, Fair Value | 1,118 |
Held-to-maturity, Fair Value, Totals | $ 1,118 | $ 1,309

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- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing in next rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. + References No definition available. + Details
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- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity), with single maturity date and allocated without single maturity date, maturing in second through fifth rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. + References No definition available. + Details
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