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Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) - USD ($) $ in Thousands | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
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Investments, Debt and Equity Securities [Abstract] |  |  |
Amortized Cost, Due in one year or less | $ 1,570,480 |  |
Amortized Cost, Due after one year through five years | 8,746,271 |  |
Amortized Cost, Due after five years through ten years | 4,395,236 |  |
Amortized Cost, Due after ten years | 2,143,998 |  |
Amortized Cost, Mortgage-backed securities | 1,859,384 |  |
Total investment in fixed maturity securities, Amortized Cost | 18,715,369 |  |
Fair Value, Due in one year or less | 1,527,090 |  |
Fair Value, Due after one year through five years | 8,299,945 |  |
Fair Value, Due after five years through ten years | 4,119,772 |  |
Fair Value, Due after ten years | 1,975,763 |  |
Fair Value, Mortgage-backed securities | 1,669,056 |  |
Total investments in fixed maturity securities, Fair Value | 17,591,626 |  |
Held-to-maturity, allowance for credit loss | $ 114 | $ 387 | $ 798

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