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Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands | Sep. 30, 2022 | Jun. 30, 2022 | Dec. 31, 2021 | Sep. 30, 2021 | Jun. 30, 2021 | Dec. 31, 2020
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Investments, Debt and Equity Securities [Abstract] |  |  |  |  |  |
Amortized Cost, Due in one year or less | $ 1,744,722 |  |  |  |  |
Amortized Cost, Due after one year through five years | 8,461,536 |  |  |  |  |
Amortized Cost, Due after five years through ten years | 4,388,799 |  |  |  |  |
Amortized Cost, Due after ten years | 2,097,205 |  |  |  |  |
Amortized Cost, Mortgaged-backed securities | 1,775,278 |  |  |  |  |
Total investments in fixed maturity securities, Amortized Value | 18,467,540 |  |  |  |  |
Fair Value, Due in one year or less | 1,691,497 |  |  |  |  |
Fair Value, Due after one year through five years | 7,982,948 |  |  |  |  |
Fair Value, Due after five years through ten years | 4,041,900 |  |  |  |  |
Fair Value, Due after ten years | 1,923,272 |  |  |  |  |
Fair Value, Mortgage-backed securities | 1,594,869 |  |  |  |  |
Total investments in fixed maturity securities, Fair Value | 17,234,486 |  | $ 16,614,118 |  |  |
Cumulative effect adjustment resulting from changes in accounting principles | $ 121 | $ 127 | $ 387 | $ 396 | $ 453 | $ 798

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