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Summary of Amortized Cost and Estimated Fair Value Through Fixed Maturities (Detail) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Due in one year or less, Amortized Cost | $ 43,213 |
Due in one year through five years, Amortized Cost | 218,132 |
Due in five years through ten years, Amortized Cost | 242,488 |
Due in ten years through fifteen years, Amortized Cost | 27,757 |
Due after fifteen years, Amortized Cost | 65,300 |
Fixed maturities, Amortized Cost | 1,260,439 | $ 1,231,568
Due in one year or less, Estimated Fair value | 43,642 |
Due in one year through five years, Estimated Fair value | 228,538 |
Due in five years through ten years, Estimated Fair value | 252,950 |
Due in ten years through fifteen years, Estimated Fair value | 29,337 |
Due after fifteen years, Estimated Fair value | 70,163 |
Fixed Maturities, estimated fair value | 1,303,775 | 1,253,159
Mortgage Backed Securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 393,325 |
Fixed maturities, Amortized Cost | 393,325 | 325,448
Estimated Fair value | 401,084 |
Fixed Maturities, estimated fair value | 401,084 | 328,374
Asset-backed Securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 137,953 |
Fixed maturities, Amortized Cost | 137,953 | 168,020
Estimated Fair value | 139,150 |
Fixed Maturities, estimated fair value | 139,150 | 168,537
Commercial Mortgage-Backed Securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 132,271 |
Fixed maturities, Amortized Cost | 132,271 | 183,944
Estimated Fair value | 138,911 |
Fixed Maturities, estimated fair value | $ 138,911 | $ 188,104

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