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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, 2023 | Dec. 31, 2022 | Dec. 31, 2021
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Investments, Debt and Equity Securities [Abstract] |  |  |
Amortized Cost, Due in one year or less | $ 1,937,329 |  |
Amortized Cost, Due after one year through five years | 9,608,345 |  |
Amortized Cost, Due after five years through ten years | 4,368,259 |  |
Amortized Cost, Due after ten years | 2,568,038 |  |
Amortized Cost, Mortgage-backed securities | 2,433,231 |  |
Total investment in fixed maturity securities, Amortized Cost | 20,915,202 |  |
Fair Value, Due in one year or less | 1,860,397 |  |
Fair Value, Due after one year through five years | 9,327,652 |  |
Fair Value, Due after five years through ten years | 4,260,217 |  |
Fair Value, Due after ten years | 2,463,851 |  |
Fair Value, Mortgage-backed securities | 2,269,430 |  |
Total investments in fixed maturity securities, Fair Value | 20,181,547 |  |
Held-to-maturity, allowance for credit loss | $ 43 | $ 114 | $ 387

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