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Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands | Mar. 31, 2023 | Dec. 31, 2022 | Mar. 31, 2022 | Dec. 31, 2021
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Investments, Debt and Equity Securities [Abstract] |  |  |  |
Amortized Cost, Due in one year or less | $ 1,568,436 |  |  |
Amortized Cost, Due after one year through five years | 8,864,666 |  |  |
Amortized Cost, Due after five years through ten years | 4,287,881 |  |  |
Amortized Cost, Due after ten years | 2,203,296 |  |  |
Amortized Cost, Mortgaged-backed securities | 2,064,073 |  |  |
Total investments in fixed maturity securities, Amortized Value | 18,988,352 |  |  |
Fair Value, Due in one year or less | 1,528,990 |  |  |
Fair Value, Due after one year through five years | 8,503,127 |  |  |
Fair Value, Due after five years through ten years | 4,084,883 |  |  |
Fair Value, Due after ten years | 2,076,887 |  |  |
Fair Value, Mortgage-backed securities | 1,901,181 |  |  |
Total investments in fixed maturity securities, Fair Value | 18,095,068 | $ 17,591,626 |  |
Cumulative effect adjustment resulting from changes in accounting principles | $ 107 | $ 114 | $ 378 | $ 387

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