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Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) - USD ($) $ in Thousands | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
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Investments, Debt and Equity Securities [Abstract] |  |  |
Amortized Cost, Due in one year or less | $ 1,744,361 |  |
Amortized Cost, Due after one year through five years | 9,337,291 |  |
Amortized Cost, Due after five years through ten years | 3,966,371 |  |
Amortized Cost, Due after ten years | 4,013,827 |  |
Amortized Cost, Mortgage-backed securities | 3,949,024 |  |
Total investment in fixed maturity securities, Amortized Cost | 23,010,874 |  |
Fair Value, Due in one year or less | 1,719,948 |  |
Fair Value, Due after one year through five years | 9,059,599 |  |
Fair Value, Due after five years through ten years | 3,902,808 |  |
Fair Value, Due after ten years | 3,949,220 |  |
Fair Value, Mortgage-backed securities | 3,767,851 |  |
Total investments in fixed maturity securities, Fair Value | 22,399,426 |  |
Held-to-maturity, allowance for credit loss | $ 25 | $ 43 | $ 114

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