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Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Jun. 30, 2025 | Dec. 31, 2024 | Sep. 30, 2024 | Jun. 30, 2024 | Dec. 31, 2023
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Investments, Debt and Equity Securities [Abstract] |  |  |  |  |  |
Amortized Cost, Due in one year or less | $ 1,725,973 |  |  |  |  |
Amortized Cost, Due after one year through five years | 8,643,436 |  |  |  |  |
Amortized Cost, Due after five years through ten years | 3,941,977 |  |  |  |  |
Amortized Cost, Due after ten years | 5,771,357 |  |  |  |  |
Amortized Cost, Mortgaged-backed securities | 5,053,558 |  |  |  |  |
Total investments in fixed maturity securities, Amortized Value | 25,136,301 |  |  |  |  |
Fair Value, Due in one year or less | 1,709,614 |  |  |  |  |
Fair Value, Due after one year through five years | 8,513,861 |  |  |  |  |
Fair Value, Due after five years through ten years | 3,971,308 |  |  |  |  |
Fair Value, Due after ten years | 5,782,835 |  |  |  |  |
Fair Value, Mortgage-backed securities | 4,971,476 |  |  |  |  |
Total investments in fixed maturity securities, Fair Value | 24,949,094 |  | $ 22,399,426 |  |  |
Cumulative effect adjustment resulting from changes in accounting principles | $ 11 | $ 14 | $ 25 | $ 30 | $ 34 | $ 43

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