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Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Jun. 30, 2023 | Dec. 31, 2022 | Sep. 30, 2022 | Jun. 30, 2022 | Dec. 31, 2021
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Investments, Debt and Equity Securities [Abstract] |  |  |  |  |  |
Amortized Cost, Due in one year or less | $ 1,818,820 |  |  |  |  |
Amortized Cost, Due after one year through five years | 9,180,726 |  |  |  |  |
Amortized Cost, Due after five years through ten years | 4,308,202 |  |  |  |  |
Amortized Cost, Due after ten years | 2,609,192 |  |  |  |  |
Amortized Cost, Mortgaged-backed securities | 2,315,785 |  |  |  |  |
Total investments in fixed maturity securities, Amortized Value | 20,232,725 |  |  |  |  |
Fair Value, Due in one year or less | 1,742,384 |  |  |  |  |
Fair Value, Due after one year through five years | 8,704,308 |  |  |  |  |
Fair Value, Due after five years through ten years | 4,074,956 |  |  |  |  |
Fair Value, Due after ten years | 2,436,157 |  |  |  |  |
Fair Value, Mortgage-backed securities | 2,079,344 |  |  |  |  |
Total investments in fixed maturity securities, Fair Value | 19,037,149 |  | $ 17,591,626 |  |  |
Cumulative effect adjustment resulting from changes in accounting principles | $ 48 | $ 53 | $ 114 | $ 121 | $ 127 | $ 387

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