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Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands | Mar. 31, 2022 | Dec. 31, 2021 | Mar. 31, 2021 | Dec. 31, 2020
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Investments, Debt and Equity Securities [Abstract] |  |  |  |
Amortized Cost, Due in one year or less | $ 1,472,865 |  |  |
Amortized Cost, Due after one year through five years | 7,903,303 |  |  |
Amortized Cost, Due after five years through ten years | 4,029,040 |  |  |
Amortized Cost, Due after ten years | 1,982,273 |  |  |
Amortized Cost, Mortgaged-backed securities | 1,450,844 |  |  |
Total investments in fixed maturity securities, Amortized Value | 16,838,325 |  |  |
Fair Value, Due in one year or less | 1,461,188 |  |  |
Fair Value, Due after one year through five years | 7,756,952 |  |  |
Fair Value, Due after five years through ten years | 3,898,727 |  |  |
Fair Value, Due after ten years | 1,927,232 |  |  |
Fair Value, Mortgage-backed securities | 1,389,515 |  |  |
Total investments in fixed maturity securities, Fair Value | 16,433,614 | $ 16,614,118 |  |
Cumulative effect adjustment resulting from changes in accounting principles | $ 378 | $ 387 | $ 730 | $ 798

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